1Leonard

How do you mean? How it is stored in the MDL file, or something else? There's nothing special in general terms.

Nothing special like DP says, but a quick run-down is this:

Each vertex has a UV position (like 0.99 0.99) which is an XY point on the texture.

Each vertex in a face has one of those UV coordinates, and the space that face uses is calculated between those three XY coordinates.

Lastly, each of those coordinates is a percentage between 0.0 and 1.0, which is why the texture doesn't always benefit from being upscaled.