Patches and similar updates.

They are not version-dependent. The format of def files, psx files, and txt tables has not changed from version to version; only the content has changed. Of course, changing the game's code would require a lot of expertise. However, WOG is simply more open to modifications due to its engine and ERM.
It is absolutely possible to import a def file created for another game and view it in-game, even in RoE. People create mods for SOD (and probably even AB) and, of course, use the tools developed by WOG enthusiasts.