The actor/wall-in-question's shade. Shade is available for the sprite structure and wall structure. Legal values are between -128 and 127 with 0 being default brightness, positive/higher values being darker and negative/lower values being brighter.
Typically 32 is enough to make a tile being displayed as pitch black, and higher values only matter for Sector Effectors that increase the lighting. Values lower than zero have no effect on the tile itself, instead it increases the visibility. For tiles displayed onscreen only values ranging from 0 to 32 are relevant.