Sound Devices MixPre Series firmware – (v1.53) provides a firmware update for previously released versions. It is highly recommended that users of MixPre-3, MixPre-6 and MixPre-10T update to the latest version of firmware available as soon as possible.
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.
This is a collection of sound mixer/recorder I can find/remember/google so please do comment below if you want another one added. Feel free to bookmark this page for easy manual reference in the future.