I just noticed today that in a render layer node where I have every check box turned on, I have four undocumented render layers that don't appear to have any check boxes, but are being output anyway:
tgSurfIndirectDiffRefl
tgSurfIndirectSpecRefl
tgSurfIndirectDiffTrans
tgSurfIndirectSpecTrans
I'm pretty sure I understand what the words mean, but when I look at the actual images they are producing, I can't understand how to make any use of them, as the first three appear to be using a single channel, and the fourth is pure black in all channels. I am using version 4.4.49.