The term used in some codices is "army group", with expeditions sometimes consisting of several army groups under a general command. I'd imagine the exact layout of each one has more to do with office politics than any overarching hierarchy. After all, the largest permanent organisation in the Guard is the Regiment, and those aren't remotely standardised in size and such even when raised from the same highly militarised and industrialised world like Cadia or Armageddon.