.rel

This file type is used by Nintendo in several games to allow for extra code to be put in Wii games.

=Usage in NSMBW= New Super Mario Bros. Wii (and all its mods) uses four .rel files located in the  folder of the game compressed to the .LZ format:
 * d_profileNP.rel.LZ (Contains Profile data for all actors in the game)
 * d_basesNP.rel.LZ (Contains code related to actors that does miscellaneous things)
 * d_enemiesNP.rel.LZ (Contains code related to actors that found in levels, mostly enemies)
 * d_en_bossNP.rel.LZ (Contains code related to actors that handle bosses)

Although these files are compressed to the .LZ format, if you decompress them and leave them decompressed (while not keeping the compressed versions around), the game will load them just fine.

=File Format Documentation= Not wanting to copy and paste it, here's a link to an article on the Custom Mario Kart Wiiki that documents this file format quite well. Considering it was made with the intent of documenting Mario Kart Wii's StaticR.rel file, some details may differ between this article and what's found in NSMBW's rel files.