These functions set the 'weapon light'. This is a slight illumination of the entire level.
A_Light0, A_Light1 and A_Light2 set this to values 0, 1 and 2 respectively. 0 means off and 2 means bright.
A_LightInverse makes the screen use an inverted greyscale mode. Note that this is not specifically Doom's invulnerability effect, but rather the flash from firing Strife's Sigil. This should not be confused with the invulnerability palette, as other Doom engine games' invulnerability effects (specifically Heretic's gold palette) are not shown when using this function.
This is the Flash state from Strife's Sigil .
Flash: SIGF A 4 Bright A_Light2 SIGF B 6 Bright A_LightInverse SIGF C 4 Bright A_Light1 SIGF C 0 Bright A_Light0 Stop