Now the final solution of Rapaire's question by an Infantry officer whose ancestors have fought in the trenches:

Stuetzpunkt is a stronghold, sometimes as a bunker = "pill box", and stollen is a mine = a tunnel under the surface, as Wolfgang stated.
The strongholds were used to reinforce certain sections of the trench net, manned by machine guns, trench mortars, and sometimes "Infanteriebegleitgeschuetze" (guns accompanying Infantry, i. e. not behind, but in the line).

When a part of the trench system was covered it was called dugout, mostly for housing of CPs, medics, telephone exchanges or living rooms for squads. Covered trenches weren't suited for the fighting by musketry.