On platforms like the Wii, .rel files are specific "relocatable" modules used to load extra code—such as gameplay logic in Mario Kart Wii —without increasing the initial memory footprint.
If you are seeing an error like REL/Relocation.h or a missing rel.dll , it is almost certainly related to a software conflict or a missing dependency:
In the modding community, "rel" often refers to Relocation logic. Tools like the Address Library for SKSE Plugins use these files to map memory addresses so that mods remain compatible even after the game receives an official update.