Shooters may not target bases contributing to frontal contact.
So to be 100 % sure
A 4 rank deep phalanx in combat : only the 3rd base may be targetted even from the rear as the 1st,2nd and 4th ranks contribute to frontal melee
A 2 rank deep long spear formation in combat: the second rank cannot be targeted as it contributes to melee even if the shooter is in the rear
Stranger bow : a 2 rank deep PA unit fighting infantry is similar to Long Spear as the unit needs bith ranks to have a + but if the same unit fights cavalry, as it only needs one rank to have a + the rear rank becomes a legal target

Romans in 2 ranks : the rear rank is always a legal target ..
So what is the intend of the rule ?
Only the frontal base in melee is a forbidden target and the others , even needed to have a +, are targets or not ?
Which has really strange effects