Street Fighter V skins come in two flavours: PAK (Unreal Engine package) installation method files and MM (FluffyQuack's Mod Manager) installation method files. You'll come across both these formats when downloading skins. PAK and MM files contain pretty much the same data except one of them is packaged in .PAK file and the other is a compilation of files in a folder. The main difference for the end user was how these skins were installed. With Street Fighter V 1.06 patch however Capcom has changed the way the game handles .PAK files and currently the recommended way to install both PAK and MM skins is to use a mod manager. Mod managers allow you to quickly enable and disable skins to your liking. There are two available: FluffyQuack's Mod Manager and Frosthaven's PAK Mod Manager.
FluffyQuack's Mod Manager pros and cons:
+ small file size
+ skin screenshot preview feature
+ no installation required
- supports only MM installation method.
Frosthaven's PAK Mod Manager pros and cons:
- big file size
- no skin screenshot preview feature
- installation required
+ supports both PAK and MM installation methods.