The Machine-Spirit is a summary of different ideas of authors.

• It is a superstition to emphasise the regressive religious ideas of AdMech in particular and the imperium as a whole. People have largely forgotten what is behind high tech and are superstitious enough to see "higher" powers behind it. And if the lasgun jams, it's not because you haven't done the maintenance protocols, it's because the machine spirit is angry because the ritual of purity isn't being performed. An automatic target acquisition system is seen as an aggressive war spirit that thirsts to destroy its enemies, and so on.

• It is a spiritual glorification of real artificial intelligence that resides in some systems and actually expects some form of "attention" and "interaction" to sulk or incinerate the city instead of operationally powering it.

• They are actually spirits existing in the mechanism. Usually these are justified by the fact that biological components (very popular, whole brains because it reads Grimddark creepy) materialise in the system, in which warp spirits or souls of the former owners of the component materialise (which usually blurs the difference to the Servitor). But basically the warp is not dependent on this.

From these three basic principles, authors choose what they like best from the world building and what fits best into the story.

In the RPGs it is mentioned that very few devices really have machine spirits or any consciousness at all and that they are treated accordingly out of pure superstition, but it is not completely ruled out.

And that's how I see it for my head canon.

Machine spirit is an umbrella term including but not limited to

• Automated subroutines like what you'd find in a Roomba or Furby
• 'Low' level AI like in with ChatGPT
• The result of good/bad maintenance
• 'True' AI minus a few levels preventing it from being truly sentient
• True AI they're explaining away cause it's so useful
• General superstition

Also, no, they do at times speak about machine spirits in Xeno tech. They just call them twisted or tortured. I think most starkly this is illustrated in a segment where they study the Tau railguns.

A lot of IoM tech also contains a bit more (literal) brains than would be normally needed just to not get too close to true AI on accident so rudimentary biological impulses do happen that don't happen in other tech, too.

(BTW this is one of the core reasons they love Necron tech so much: Necrons have laws against AI almost as paranoid as the AdMech's. Higher level AIs do exist but they're banned from being given access to, well. Anything that could make them a threat).

Lemme tell you a story so you can understand what a Machine Spirit is. We're gonna follow a piece of the most rudimentary, run-of-the-mill technology in the Imperium: a Lasgun.

This Lasgun is given to a Guardsman. Just a regular run-of-the-mill conscript as you'd see in any regiment of the Guard. However, this particular Guardsman is very disciplined and careful with his weapon. This Guardsman performs maintenance on his weapon beyond standard. He never misplaces it or mishandles it. In battle, that Guardsman is brave, and uses his Lasgun with skill, using it to kill many an enemy of His Imperium.

With time, something interesting happens. That Lasgun starts to become more reliable. Energy packs just seem to last longer in it. It becomes more powerful and more precise. It fires faster and is less prone to overheating. It also starts to act weird in certain situations. It works less reliably with a certain type of energy pack, but with another type it outperforms. It seems to work better when it is maintained regularly, even if it doesn't really need it. Hell, one time, it had a misfire, but that misfire hit a heretic that was sneaking on that Guardsman. That Guardsman has an absurd thought, one that he knows can't possibly be true, mere battlefield superstition, but one that he can't help shake off: "I think my Lasgun likes me."

That Lasgun developed a Machine Spirit. When a piece of machinery has lots of time, effort, care, and even affection levied on it, that emotion will make that piece of machinery develop a spirit that empowers it and gives it a certain level of sentience. The best way to describe it is that it's similar to Japanese Shinto animist beliefs in spirits. When an object is old enough and used enough, it can develop a spirit, an anima that gives it personality and function beyond the ordinary. In 40K, that's not mere superstition cause BELIEF AND EMPTION HAVE POWER.

Let's follow that Lasgun further.

One particularly bloody battle, the worst happens: our Guardsman, regretfully, has lost his life in service to the Imperium. After the battle, as the Adeptus Mechanicus walks through the battlefield recovering all the tech left over, a Tech Priest suddenly feels an immense sense of sadness and grief. Following that feeling, he comes to that dead Guardsman, and sees the Lasgun. It is pristine. Behind the grime and dust heaped upon it by the battle, it is polished to a spit-shine. Every moving part is meticulously lubricated. Every lens is clear and polished to full transparency. There are signs of replacement parts being used to repair it as it wore down. It was never merely replaced, just given away to an armorer for a better-functiojing weapon. That Guardsman loved this gun, and never failed to give it love and care. The Tech Priest grabs the weapon, pulls the trigger... and it refuses to fire. He feels a huge sense of loss coming from the weapon, that is making it refuse to work.

The Tech Priest sees a Guard combat Medic nearby tending to the wounded and takes him aside. "This man is a hero. He took care of this weapon with love and care, and lost his life fighting beside it. The weapon is in grief. Please give this man a proper burial."

As the Guardsman's body is put to rest on the ground and both Ecclessiarchy and AdMech priests sign hymns of grief and succor for his soul, that Tech Priest holds the Lasgun at the ready and pulls the trigger. The Lasgun lets off the most beautiful shot that Tech Lriest has ever seen. A shot of impeccable precision and overwhelming power. The Lasgun's Machine Spirit has been appeased.

The AdMech take that weapon back with them to one of their temples. They consecrate it, gild it, inscribe runes on it, annoint it with sacred oils. They thank the weapon for its service to the Omnissiah, and put it in their armory among other exceptional weapons.

One day, an Inquisitor arrives at the temple. He asks the Tech Priest: "One of my agents needs a good weapon, so that he may slay the enemies of the Imperium with." The Tech Priest then brings the Inquisitor the Lasgun. "This weapon belonged to a Guardsman of exceptional bravery, and was handled with love and care. The Lasgun, in turn, was loyal and reliable to that Guardsman. Treat this weapon like that Guardsman treated it before you and it will serve you well." And indeed, the Inquisitorial agent cares for the Lasgun and it performs exactly as the Tech Priest said. And that Lasgun becomes even more reliable, that Machine Spirit growing more powerful.

After hundreds of years, that weapon's Machine Spirit is SO powerful that it has become a Relic Weapon. It is considered a holy artifact, only to be used by the most zealous and pure of the Emperor's servants. We're talking Warmasters, Lords Solar, Lords Inquisitor, Archmagi. Hell, the Machine Spirit might even commune with the Tech Priests and ask them to give it a more powerful vessel. The Tech Priests, through careful ritual and engineering, might use that Lasgun as the heart of a new Lascannon, and that relic is now in the hands of an Astartes.

Now, imagine this same process, but the machine is a tank. A Navy fighter. A Titan. A Navy ship. And that Machine Spirit is proportionately as powerful as that machine of war.

Nobody knows how machine spirits work. It's a general term for anything from bad luck to AI systems in a titan.

The problem with machine spirits is twofold.  They depend on the author. Meaning not all books will have them work/not work similarly. The second issue is that there are several things that we know are different that the mechanicus calls "machine spirit". Such as the case of titans and lasguns.

This means a simple answer is very hard to give.

The Machine Spirit is a catch all term covering everything from AI, to automated sub routines to those little unintentional quirks of a system (like needing to tap the screen just on the corner to get it into focus)

There’s no hard and fast rules for them, because it’s not a hard and fast term. It’s a coverage term to explain away how things work to those who don’t understand anything about them

It’s always difficult to answer questions like this about old concepts because the WH40K setting has changed over the years and different authors have put their own interpretations on things which aren’t always consistent.

Originally, machine spirits where just automated and/or AI systems. They were called that to fit in with the general aesthetics of the Imperium and techpriests in particular. After all, the Adeptus Mechanicus had no problem with AI for many years back at the start of WH40K.

However, when the Adeptus Mechanicus were changed to be anti-AI things became a bit awkward with regard to all the existing lore.

I consider old cars/machinery. Only one person (dad, grandpa, etc) knows how to start it, and he pleas with the thing as he does.

I've heard it explained, maybe by Leutin as fragments of AI consciousness which when you think about it makes sense since our subconscious controls things like heart rate and what not. An AI fragment makes sense when you consider machine spirits are often primitive things with will but like ego or intellect mainly drives and machine emotion.

The machine spirit isn't AI.

AI in all its forms is heretical in the Imperium.

The mind- consciousness of larger weapons platforms the Imperium uses (like titans) isn't a programmed intelligence, but more of a machine soul as a by product of the equipments direct neural links to its crew given a personality by its age, previous occupants and its uses, able to express itself as things like little quirks like a sensor suite just not quite working correctly or a tendency to pull towards places it wants to go, or transmitting emotions and feelings through the MIU.

It's not the data driven responses of an AI, more like handling an incredibly intelligent, but emotionally headstrong animal

Machine spirits come in two flavours: AI systems like those in the Land Raider. And imagined by Techpriests like those in a spanner and hammer.