I have question about uv

Firstly, it is good to know that when dealing with CAS that there are two uv map to deal with.The uv_0 and uv_1.

The uv_0 is the uv template for the Sims body, while the uv_1 is the uv template of the CAS objects and it handles all the objects that the sim wears from hair, rings, shoes, watches hats, caps, bracelets, bangles, and so on. So the uv_1 is like a layer over the uv_0. To understand more on this CAS uv_1 please see this TUTORIAL

Again, to understand more on this as regards what is what, please take a look at this TUTORIAL. If you go to the link that above, you will find the template for the sims body that gives you a guide as to what is supposed to be with respect to the CAS item you are making in relation to the Sims body. Meshing an object and applying a texture to it, is totally different from that of a CAS object. So the template comes in handy for CAS objects, to help you place the part where it to be.

As for an example, if you look at the page that the above second tutorial points to, to in Sims4studiio, the second image shows a purple colour at the top left of the template that refers to HAIR. What it means is that when you are creating a Sims hair, you have to map the uv of the hair to that location and at the same time apply the texture of the hair to that same location. Hoping this helps you out to a more understanding of the questions that you asked.