BBCard?
I thought I had something somewhere that mentioned what the different file types were, but on a quick skim then it seems I don't. Since it's a separate topic I'll make it a new topic so we do have a reference.
Basically, RSH files are the bottom level of texture. Each model references one or more RSH files and the model will always use them as a last resort. The effects in the RSH also still apply once the WTP or RTX is loaded over it, but the base texture is hidden.
If you are playing a skirmish then DoW will look for a WTP file with a matching file name with "_default" appended and use that for team colouring.
If you are playing a campaign then DoW will default to RSH files, but you can also use SCAR scripts as part of the campaign to specify RTX sets to use. RTX is similar to RSH in that it is a DDS file with a fixed colour scheme, but it only contains the one image where as RSH contains the base image and some optional effects (specularity and self illumination).
Hopefully that clears things up a bit