The system and default Segments

The system and default segments are mapped to each new database device included in the on clause of an alter database command. To unmap these segments, use sp_dropsegment.

When you use alter database (without override) to extend a database on a device already in use by that database, the segments mapped to that device are also extended. If you use the override clause, all device fragments named in the on clause become system/default segments, and all device fragments named in the log on clause become log segments.