You may need to reduce the scope of a segment if it includes database devices that you want to reserve exclusively for other segments.
For example, if you add a new database device that is to be used exclusively for one table, reduce the scope of the default and system segments so that they no longer point to the new device.
Use sp_dropsegment to drop a single database device from a segment, reducing the segment’s scope.
sp_dropsegment drops only the given device from the scope of devices spanned by the segment. You can also use sp_dropsegment to remove an entire segment from the database.
See the Reference Manual: Procedures.
sp_dropsegment bigseg, pubs2, pubs_dev2