In DB2 for OS/390 you can have many databases in a subsystem. The database is an administrative level only in DB2 for OS/390 and does not affect DML (insert, update, delete, select).
You can grant DBADM access at the database level, but not insert, update, delete, or select access. In DB2 for OS/390, the database is more like a schema in DB2 for other platforms.