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You can have a look on wikipedia what RGBE is:
And if you take a look in the source code of the following file:
Source\Runtime\ImageCore\Private\ImageCore.cpp
You’ll find switch cases with case ERawImageFormat::BGRE8 that points to functions such as FLinearColor::ToRGBE and FColor::FromRGBE which implies that BGRE8 is related to RGBE. Also there is the following comment on those 2 functions, which seems to explain what the RGBE 8888 encoding does.
/**
* Converts from a linear float color to RGBE as outlined in Gregory Ward's Real Pixels article, Graphics Gems II, page 80.
* Implementation details in https://cbloomrants.blogspot.com/2020/06/widespread-error-in-radiance-hdr-rgbe.html
*/