A surcharge is whatever loads the ground surface beyond the soil itself, and behind a retaining wall it converts directly into extra horizontal push: a uniform surcharge q contributes a rectangular pressure block of intensity K·q from top to bottom of the wall — total thrust Pq = K·q·H acting at H/2 — superposed on the triangular soil pressure and any hydrostatic water pressure. A full wall check resolves the driving forces from earth pressure, surcharge, and water against the resisting wall and soil weight plus passive resistance, producing the overturning, sliding, and bearing factors of safety.
The pressure blocks keep separate addresses, which is how the resultant is located: the triangular soil diagram acts at one third of the wall height above the base, the surcharge rectangle at mid-height, and a water wedge at one third of the submerged height, so the combined line of action is their area-weighted average — the height the overturning moment is computed about.
The same word does different duty under a footing: there, the surcharge is the overburden beside the foundation, q = γ·Df, and it helps rather than pushes — it is the q·Nq term of the bearing-capacity equation, worth 27 × 18.40 = 496.8 kPa of the worked strip footing’s 1,381.6 kPa ultimate capacity, and the quantity that net allowable bearing capacity subtracts back out.