Weight issue: under arm glitches

Hi. Since sims spend most of their time standing with arms down, clothing with loose sleeves should be simulated in md with arms down as well. In your case you might be able to fix this by editing weights, so try that first.
Also, please choose the thread title that gives people the idea of what you are dealing with. This will attract people who know how to fix it ( and so will attaching images of the problem) and you will receive the answer faster. Otherwise you will have to wait until someone gets to their computer and checks the file. Also, straightforward title can help those with similar issue to find the solution by googling. I changed the title to give you the idea
A few more things:
- if you are making a full-body outfit, start the package from the full body outfit, not top. They are affected by different bones which cal lead to severe distortions. If you won't see them, other people using your cc may;
- 1/4 of the polycount accounts for buttons. For that reason it is best to not use md's default ones. Either use custom ones or retopologize them. If you will delete the threads and the backside, then collapse the holes, you'll reduce the polycount by ~5k tris.

- when you zoom out in s4s, see the skin-colored lines along seams? It will get worse in live mode in-game. You need to bake textures with at least 7 px margin.  You can add it in the image editing software to not re-bake the textures;
- when closing holes, unwrap the newly created geometry. If that hole is not visible from any angle, there is no need to close it at all;
- you should check how the mesh behaves in-game, then make textures, lods, swatches, import thumbnails, etc. If something is wrong with how the mesh is shaped or adapted, you might have to re-do some of these things.