Depends on the management class.

If the data is bound to a management class that points to a disk pool, but
the disk pool has a NEXTSTGPOOL pointing to tape (i.e., the disk pool
migrates to tape after it fills):

The data goes to the disk pool.  If the disk pool is too full to hold the
incoming data, and MAXNUMMP is set to 0, the backup will fail with an error.
If MAXNUMMP is set to 1 or more, the backup will wait until a tape drive
(mount point) is available.

If the data is bound to a management class that points directly to a tape
pool and MAXNUMMP is set to 0, the backup will fail with an error.  If
MAXNUMMP is set to 1 or more, the backup will wait until a tape drive (mount
point) is available.