These are what different recording formats on the Sound Devices 688 / 633 and 664.
The different modes are Wav poly / wav poly (ISO Only) / wav poly (LR Only) / wav poly (X1X2) / MP3 (LR) / MP3 (X1X2) / wav mono / Wav Mono (ISO only). All of these only work if you record enable the relevant tracks.
Wav poly = Records all tracks and mixes that you record enable in 1 file
Wav poly (ISO Only) = This records only the tracks that are record enabled but not any mixes or Aux tracks.
Wav poly (LR Only) = This records just the left and right main mixes and nothing else.
Wav poly (X1 X2) = This only records the aux tracks called X1 and X2
MP3 (LR) = Records the left and right main mix to a stereo MP3 file
MP3 (X1 X2) = Records Aux 1 / 2 as a stereo MP3 file
Wav Mono = Records all tracks as separate files from each other.
Wav Mono (ISO only) = Records only ISO tracks as separate mono files.