Diffuse

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 it has shading and specular done during rendering (using normalmap and specular map).

In Darkplaces, there is no special texture slot for a diffuse, it is a 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 a generic color texture.