IBM TOTAL STORAGE DS8000 - Manual (Page 147)

CMUS00033E Select at least one volume.
Explanation
At least one volume must be selected before you can create a Global Mirror
relationship. No volume has been selected.
Action
Select at least one volume, and then resubmit the task.
CMUS00034E No session numbers are available for the selected
volumes.
Explanation
No available sessions exist because all 256 possible session numbers have been
used already.
Action
To make session numbers available, remove one or more of the storage images in
the configuration by deselecting all of the volumes in that storage image.
Chapter 4. DS Storage Manager Copy Services messages 127