Quake III Nation has a reply from id Software's Kevin Cloud to their query about how custom textures will be handled in Quake III Arena, which is basically the same as in Quake II, in a separate .pak file (*though he points out the textures are in the standard .tga format rather than Q2's proprietary .wal format). The post also mentions "shaders files," which are "short scripts that define the properties of a surface or brush content as it appears and functions in the game."