User or system data on migration level 1 volumes

Migration level 1 volumes, once defined to DFSMShsm, are known and used as DFSMShsm-owned volumes. That expression implies, among other things, that when DFSMShsm starts using such a volume, it determines the space available and creates its own record of that free space. For reasons of performance, DFSMShsm maintains that record as it creates and deletes its own migration copies, backup versions, and so on; DFSMShsm does not keep scanning the VTOC to see what other data sets may have been added or deleted.

Restrictions: The point is that your installation can store certain types of user or system data on migration level 1 volumes as long as you keep certain restrictions in mind:

Such data sets cannot be SMS-managed, because these volumes cannot be SMS-managed.
Once such a data set is allocated, do not change its size during an DFSMShsm startup.
Do not request DFSMShsm to migrate or (except perhaps as part of a full dump of such a volume) back up such data sets.

Given that you maintain these restrictions, you can gain certain advantages by sharing these volumes with non-DFSMShsm data:

A given amount of level-1 storage for DFSMShsm can be spread across more volumes, reducing volume contention.
Since only one SDSP data set can be defined per volume, the number of such data sets can be increased.