An as400 subsystem is described by IBM as follows
The subsystem is where work is processed on the system. A subsystem is a single, predefined operating environment through which the system coordinates the work flow and resource use. The system can contain several subsystems, all operating independently of each other. Subsystems manage resources.
All jobs, with the exception of system jobs, run within subsystems. Each subsystem can run unique operations. For instance, one subsystem may be set up to handle only interactive jobs, while another subsystem handles only batch jobs. Subsystems can also be designed to handle many types of work. The system allows you to decide the number of subsystems and what types of work each subsystem will handle.
The run-time characteristics of a subsystem are defined in an object called a subsystem description. For example, if you want to permanently change the amount of work (number of jobs) coming from a job queue into a subsystem you only need to change the job queue entry in the subsystem description.
The controlling subsystem
The controlling subsystem is the interactive subsystem that starts automatically when the system starts, and it is the subsystem through which the system operator controls the system via the system console. It is identified in the Controlling subsystem/library (QCTLSBSD) system value.
IBM® supplies two complete controlling subsystem descriptions: QBASE (the default controlling subsystem) and QCTL. Only one controlling subsystem can be active on the system at any time.
When the system is in the restricted condition, most of the activity on the system has ended, and only one workstation is active. The system must be in this condition for commands such as Save System (SAVSYS) or Reclaim Storage (RCLSTG) to run. Some programs for diagnosing equipment problems also require the system to be in a restricted condition. To end this condition, you must start the controlling subsystem again.
Note: There is also a batch restricted state in which one batch job can be active.
When all of the subsystems, including the controlling subsystem are ended, a restricted condition is created. You can end each subsystem individually or you can use the ENDSBS SBS(*ALL) OPTION(*IMMED).
Important: The system cannot reach the restricted state until there is only one job remaining in the controlling subsystem. Sometimes it may appear as though there is a single job remaining, but the system does not go into the restricted state. In this case you need to verify that there are no suspended system request jobs, suspended group jobs, or disconnected jobs on the remaining active display. Use the Work with Active Jobs (WRKACTJOB) command and press F14=Include to display any suspended or disconnected jobs. If these jobs exist, you need to end them in order for the system to reach the restricted state. The ENDSYS and ENDSBS functions will send a CPI091C information message to the command issuer when this condition is detected.
A subsystem description is a system object that contains information defining the characteristics of an operating environment controlled by the system. The system-recognized identifier for the object type is *SBSD. A subsystem description defines how, where, and how much work enters a subsystem, and which resources the subsystem uses to perform the work. An active subsystem takes on the simple name of the subsystem description.
Like a set of detailed blueprints, each subsystem description is unique, containing the specific characteristics that describe the subsystem. The description includes where work can enter the subsystem, how much work the subsystem can handle, how much main storage (memory) will be used, and how quickly jobs in the subsystem can run.
You can use a subsystem description supplied with your system (with or without making changes to it), or you can create your own.
Subsystem description attributes
Subsystem description attributes are common overall system attributes. When you create a subsystem, the first step is to define the subsystem attributes.
Work entries identify the sources where jobs can enter a subsystem. Specific types of work entries are used for different types of jobs. Work entries are part of the subsystem
ibm as400 manuals v5r4
The routing entry identifies the main storage subsystem pool to use, the controlling program to run (typically the system-supplied program QCMD), and additional run-time information (stored in the class object). Routing entries are stored in the as400 subsystem description.