These groups are used in generic meshes.
They tell how many textures go on one mesh part and wich ones. The order of the listed textures is quite important there^^
for eg render group 1_ wants 6 textures they have to be listed like : diffuse, lightmap, bump, mask, minibump1, minibump2
but this changes with the render groups for eg 6_ only needs 2 textures listed like: diffuse, bump
but well mesh 1 doesn't have to be render group 1. you can use every render group you want on any mesh you want....