Instances and subsystems.
In DB2 ULWO, an instance provides an independent environment where database objects can be created and applications can be run against them. When an instance is created, links to the DB2 code are generated. Several instances can be created in the same machine. In Windows platforms, you can install only one version of DB2 at a given fixpack level. Thus, all instances created in DB2 UDB for Windows will be linked to the same DB2 code. In UNIX platforms, you can install several versions of DB2 in the same machine because they are installed in different paths; however, only one fixpack level per version is allowed. Thus, in DB2 UDB for UNIX, you may have several instances linking to different code sets.

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, but I don’t cover that topic in this article.)

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.

Naming in DB2 ULWO names you may need are:

An instance name.
An instance name uniquely identifies an instance in your machine. As described in Directing commands, you use the instance name to direct commands to a particular DB2 instance.
One or many database names
Unlike DB2 S/390, a client connects to a particular database rather than a subsystem.
When connecting to a database using TCP/IP, you will also need the TCP/IP address and port for the instance. Other network protocols will need other information.

In DB2 S/390, several names are used to identify a subsystem:

Subsystem ID (ssid)
Because DB2 is a subsystem of MVS, it will have a subsystem name or subsystem ID (ssid) that MVS can use to identify it. The default ssid is DSN1. The command prefix identifies the particular DB2 subsystem that you connect to. You do not connect to a particular database. You are automatically able to access any database in DB2 to which you are authorized.

The location name.
This is the name specified when using the CATALOG DCS DATABASE command from a DB2 Connect machine, which refers to it as the target database name. It can have from 1 to 16 characters.

The LU name.
This is the name by which VTAM can recognize the local subsystem. The unique name must be unique within the network of connected systems and can have from one to eight characters. When connecting to a DB2 S/390 subsystem from a DB2 ULWO or DB2 S/390 client using TCP/IP, the TCP/IP address, the port for the subsystem and the location name are needed.