Need help with weight transfer

Hi. You do not need 2.7 to transfer weights, you can delete it. For the future, do not watch tutorials that tell you to get 2.7 just to transfer weights and vertex paint, those are made by beginners and are full of identical mistakes.
Another "for the future", please always provide the files (blend, package) so that we would know what exactly is wrong.

- find a reference of the same type (skirt-like bottom or pants/legs) and length, clone it, export the blend;
- append it in the scene with your garment or vice versa, join all parts of the reference if needed, rename to Reference;
- make sure both Reference and your garment are visible and selectable;
- select Reference, shift-select your garment, switch to Weight paint, choose Transfer weights, choose Nearest Face interpolated and By Name;
- choose Clean, choose All groups;
- choose Limit total.

For the item to morph as intended, it must receive all the data from the area of the body it covers. The bracelet covers the wrist, so to follow the wrist instead of flying or clipping into it, the bracelet must receive all the data responsible for moving and morphing from the wrist. The same exact logic applies to the dress except for it is a dress, not a nude body. Its bottom is shaped like a skirt, not legs, and those two things must move and morph differently.

In your blend file switch to Weight paint, select b__CAS_L_Breast__. See how below the breasts everything smears and below the navel there's nothing? Whatever you transferred those weights from was not a dress of appropriate length.

Pretty much everything is done wrong, so you need to restart.
- go back to MD, select both patterns, remove Remesh (do not use it, it makes the topology worse). Then either change the topology to quads or retopologize the mesh;
- export as thin and weld, no internal shapes or textures;
- clone a similarly shaped maxis item. You can type "chemise" in the search bar, it would be perfect for this. So clone it and export the blend;
- import your dress, transfer weights, uv_1, and vertex paint to it from the reference;



- expand the rig, make top and bottom visible and selectable, select top, in 3d view press Shift D, then Esc. Do the same with bottom. Select top.001, shift-select bottom.001, press Ctrl J to join. This will be the body;

- select the joined mesh, disable Limit selection to visible and delete all parts of the body the dress covers. The more of it is deleted, the more space you will have for the garment's uvs;

- select the dress, in its uv_0 select all uvs and move them outside of the uv space. Join the dress with the body. In uv_0 select the uvs of the garment and assign this template. The uvs of the garment will stretch vertically, so bring them back to their original size with S Y 0.5. In the blend you shared they are shrank too much, you only need to do it in half. Then you can put them is the free space;
- close the hole at the bottom and unwrap. Close holes in the sleeve and cleavage areas;

- assign 0000 as the cut and GEOM as type, save, import in s4s, test in-game.

Thank you so much for this detailed torublesolving post. I have been creating my own meshes for personal use for a few years now, and two days ago I've encountered a weight issue that was a first and I didn't know how to solve it. I was going crazy and didn't want to trouble anyone on s4studio cause I was pretty sure that there would've been an answer. The data transfer thing did it for me, since I've been already using the clean,and limit method up until now and had 0 issues with weights.

Thank you sccp for asking for help which triggered this post and thank you mauvemorn for ALWAYS trying to help us in this site without missing a beat. A pillar to this community for us newcomers and ogs as well.