Hiya,
Not in front of the H9000 or in the studio at the moment.  However I’m assuming the auto/64 setting is to choose between 56 channel and 64 channel mode of the MADI.

I’d say most of the time you’ll see 64 channel mode streams.

Some devices run natively in 56 channel mode and use the last 8 channels for pre-amp control data. Other use case is one can run in 56 channel mode and be able to Vari-Speed further up and down than in 64 channel mode.
At 96kHz all these numbers half.

hope this helps.  All corrections and suggestions to the above accepted!!