I understand that the correct channel naming for 5.1 layouts is: FL, FR, FC, LFE, SL, SR. The last two being "surround", not "back". It's 7.1ch that adds the back channels as BL and BR (or whatever letters are the standard). SL and SR are actually in the "middle" and the added two in 7.1 are placed at the back. Like this diagram. Everything makes sense and we're happy.
BUT, in the old days the common belief was that 5.1 had the "back" channels (BL and BR in the above diagram) and 7.1 added the "side" ones (the word "surround" didn't appear at all). Idk if that was some kind of old standard before the advent of 7.1 or it was wrong all along.
Thing is, some programs still carried the old standard into the era of the "surround" naming convention. Back when I made my own BD remuxes I had quite a lot of discs with LPCM 5.1 which I converted to FLAC. I don't remember which program(s) I used, ffmpeg, foobar probably, but all my FLACs ended up as "L R C LFE Lb Rb". Those last two should really be "Ls Rs". Assuming the program(s) didn't mess with the channels order then it isn't a real issue, the layout is correct, but is there a way of renaming the channels? If I re-encode the FLAC the channel layout carries along and I don't see how to alter it.
Nowadays I convert all LPCM with eac3to which writes the proper channel data as "L R C LFE Ls Rs".
For FLAC you can use metaflac
metaflac.exe --remove-tag=WAVEFORMATEXTENSIBLE_CHANNEL_MASK --set-tag=WAVEFORMATEXTENSIBLE_CHANNEL_MASK=0x60f file.flac
Also, some software can remap channels during conversion, for example, foobar2000 with DSP Matrix mixer.
Notice, that some decoders which don't support reading WAVEFORMATEXTENSIBLE_CHANNEL_MASK from FLAC can still treat files as 5.1 back anyway.