Tes5Mod:Mod File Format/REGN
REGN are "regions", but not in the locational sense (those are LCTN). Each region ties together ambient information (weather/music/etc), presumably so they can be dropped into settings and provide a complete ambience.
Between the thick line are repeatable field sets, RPLI/RPLD come in pairs, and every occurrence of the bottom fields is preceded by an RDAT (sole exception: RDM0/RDSA can be under same RDAT). RDAT can also appear by itself (though rare).
|+||RCLR||region color||rgb||Map Color (unknown where used)|
|-||WNAM||world name||formid||World (WRLD).|
|-||RPLI||uint32||Edge Fall-off (World Units)|
|-||RPLD||data for "pl"?||struct||series of 8-byte structs, vary in length from 32 to 172 look like floats, coords?|
|-||RDAT||data header?||struct||8-byte struct
|-||RDMO||music||formid||Music (MUSC) formid. Can appear with RDSA under same RDAT or on its own.|
|-||RDSA||sound data||struct||12-byte struct, total count arbitrary
|-||RDWT||weather data||struct||12-byte struct, total count arbitrary|
|-||RDOT||Region generator objects||struct||Used for region generation: always 0 length in game files.
52-byte struct, total count arbitrary.
|-||ICON||shader?||zstring||path to .dds Trees\CanopyShadow.dds|
|-||RDMP||Region Name?||lstring||Map Name|