Hi Daniel,
MaxDataVolumes only shows the number of data volumes you can have without restarting the MaxDB instance. No data volume is added automatically.
AFAIK the "Automatic Data Area Adjustment" does the same as you can do manually, it adds data volumes. I guess it can usually add up to 10 data volumes without restarting the instance.
So both options are different but have related meanings.
Regards,
Simon