Diffuse

Color texture (on the left) and diffuse texture

Diffuse texture - a version of color texture applicable to Phong shading. Usually color texture contains shadowing and specular applied - info that is not required for Phong shading as shadowing and specular are done during rendering (using normalmap and specular map).

In Darkplaces, there is no special texture slot for a diffuse, it is replacing color texture. So it matters how your surface world be lit (with phong shading or just with lightmap) to determine, if should draw a diffuse texture or generic color texture.

Lighting up a non-diffuse texture with Phong shading will decrease an effect of realtime per-pixel lighting, as both calculated and baked shades will appear.

Tip: Blood Omnicide using Phong shading on all surfaces.