A mount request has been made for a volume that is defined in a storage pool, but which is currently checked out of the given library.
The server waits until it detects that the volume has been checked into the library, or the time limit expires.
Obtain the required volume, and insert it into the library by issuing a CHECKIN LIBVOLUME command. Use the SWAP=YES option of the CHECKIN LIBVOLUME command if the library is currently full; this process allows the server to select an appropriate volume to be swapped out in order to make room for the required volume.