The entrenchment rules on pg 75 say "If units are allowed to entrench ... Some army lists and scenarios might allow an army to begin the game with entrenchments ..."
A cursory look at the list shows that Opolcheniye and some Spanish units can begin the game in entrenchments.
So in a AB game who can entrench during a game - anyone, no-one, or only units that are allowed to be entrenched at the start of the game?
Cam