[Mobhack]

Fortifications happen to be a particular unit class. That is the differentiation.

That unit class is treated as a little bit permeable to HE fire, unlike tank type steel armour. So crew casualties can occur in concrete fortifications, even if they are a "tank turret" icon. Splinters are assumed to go through embrasures or whatever. That has been the design philosophy for concrete forts since SP1.

Less concrete means more chance of leakage, larger shell size has more chance.

There are no deep fortifications that are effectively impervious to fire, such as the German WW1 deep trench bunkers. So a "Somme type" week long bombardment in the SP universe would eventually clear all defenders. But that is not the focus of this game.