Most games store the same textures as duplicate objects in several packages, for performance reasons.
UPK Explorer attemps to group these identical textures together by identifying the textures using a generated name (which is group path\object name).
However there are cases where the generated Id would actually group together textures that are not duplicates.
This is where the IdRemappings.xml comes in. If no IdRemappings.xml can be found for your game under GameProfiles\IdRemappings, a new one will be generated when openining the game.
It will log all package names as well as texture names that should not be grouped together.

When generating a texture pack, the IdRemappings is copied to the texture pack folder, and TFC installer will use it to inject the correct textures.

When you open a game with UPK Explorer that has a package that is not in the mappings file, you will be warned that your IdRemappings file might not be up to date.

There are two possibilities here:
- if this is a modded package (like a new skin you downloaded), you should exclude that file from your game profile (See Game profiles instructions)
- if this is an official package (following a game update...), you should delete your IdRemappings files, and the next time you will open the game, a new one will be generated for you.