From WarzoneWiki

Why does it exist? What roles does it play?

The handlers are specified in src/data.c, and most of the low-level code is in lib/framework/frameresource.c

Resources are specified in WRF (Warzone Resource Files?), and also in the two level files (.lev). The latter are used to associate resources to multiplayer maps, for instance.

Some buildings in the game have different foundations depending on which terrain type they are in. In order to facilitate such changes, the PIE related texture pages are named “page-NUMBER-STRING”, where only “page-” and its number are kept internally, and the last loaded texture page that matches is kept in memory. That way a WRF or LEV file may specify a different set of textures for a map.