BACKUP uses the STACKER and SIZE parameters to determine whether there is enough space for the backup.
For disk backups, if it decides that you have not provided enough space, it fails the backup before actually writing any of the data.
If it decides that there is enough space to start the backup, but then runs out before it finishes (for example, if your estimate is incorrect, or if a user in another application fills up a lot of disk space while your backup is in progress), an attended backup prompts you to load a new tape, or to free up disk space. An unattended backup fails if it runs out of space.
If neither STACKER nor SIZE is specified, backup proceeds until it completes or until the tape or disk is full. If you run out of space, an attended backup prompts you to load a new tape, or to free up disk space; an unattended backup fails.