When FF7 was ported to the PC, Sony didn't allow the PC guys to use Psy-Q (The PSX development libs), so a psudo-Psy-Q emulation layer was created. There was a also a slight format shift to something a little more PC-centric, but the data pools were all designed with Psy-Q in mind. This meant that a call to "load_texture" would default to the loading of a TIM in the PSX version, but a TEX in the PC version. The accessors, (Looking up LBA addresses in the PSX version, and uncompressing the LZS, verses the PC methods of opening an LGP and loading the data) are different, but the data pools that were the end result were the same functionally.. TIM files are optimized for the strange multi-depth surface of PSX vram, while TEX files are more sane palletted formats for PC video memory. In the end when the FF7 kernel makes the call, something is loaded and the program in none the wiser.
Does that make sense?