If I install a body texture mod I am assuming that that texture is only used when the character is not wearing anything (just being in their underwear). This I can see on screen.

Now, if I equip a sleevless armor that has a skirt (like the arena rainment) I should not expect to see the body texture on the exposed skin parts (except the face) is that correct?
Because if that is the case, you would only want to install hi-rez body textures if you want to see NPCs in their underwear (or nvde).

Unless the clothing and armor mods actually use hi-rez sking textures for the exposed parts but I have never seen one mention this. I use EVE and I do not think there is a hi-rez version.

Actually, 99% of armors/clothes use body textures on parts where there is no clothes. You can check for yourself in NifSkope. There is path for texture armor and for body armor which is set to default body texture. So if you have high rez body textures, all armors showing skin will have those textures.

Thanks for the answer. Nifscope is over my head