e2a: the shadow rank thing is quite odd/interesting. Off you go down the rabbit hole, your MCM div basically promote you based on time, whereas over in the Regt they run their own scheme.
So more often than not, when you return to your original arm or corp you will have been promoted and need to update your tailoring.
It was as much to do with the pension on departure from the military. The Regiment were upfront enough at the 5/6 year point to advise - " if you stay, you are highly unlikely to be promoted beyond your present rank, (invariably JNCO), however, when you return to your parent unit you'll be 'x' rank and on a much better pension run ". Some Sqn. Cmdrs and SSMs didn't phrase it so diplomatically as one or two authors have subsequently remarked.