In DB2 S/390, a DB2 subsystem provides a separate DB2 environment similar to a DB2 ULWO instance. Several DB2 S/390 subsystems can be installed in the same machine logical partition (LPAR), and they can only communicate with each other through the Distributed Data Facility (DDF).
(Data sharing provides another way that DB2 subsystems work together)
You can install different DB2 S/390 subsystems at different versions in the same LPAR. You can also have different DB2 S/390 subsystems at the same version but with different maintenance levels installed in the same LPAR. In both of these cases, different code sets are used.
For example, you might have DB2 S/390 V6 at maintenance level 0112, DB2 S/390 V7 at maintenance level 0106, and DB2 S/390 V7 at maintenance level 0110 installed in the same machine LPAR. DB2 subsystems running with the same version and at the same maintenance level in a LPAR, are also allowed; in this case the DB2 code is shared.
In DB2 ULWO, you can direct commands to a specific instance by setting the value of the DB2INSTANCE variable (
set DB2INSTANCE=<instance name>) or “attaching” to the instance using a node previously defined (
attach to <nodename>). In DB2 S/390 you can execute DB2 commands from different places. The
START DB2 command is the only command that is normally executed from the MVS console. Because you can have different DB2 subsystems installed, a command prefix is required for MVS to know to which DB2 should this command be applied to. This command prefix can have up to eight characters. The default is -DSN1, in which DSN1 is the default MVS subsystem name for DB2.
Read complete story on DB2 ULWO (Unix, Linux,Windows and Zos) here.