Dispatchable Unit is the a unit of work, an individual piece of work.

It can be a single program, or a few programs executed in a sequence.

Each DU can own memory, locks and other resources. They operate individually and independently of other DUs.

You can think of a DU as like a thread (on a UNIX system)

Each DU executes within an address space. For example a TSO user, a batch job, a z/OS UNIX process or started task.

There will be many DUs executing within each address space. Tasks within an address space can share memory and other resources.

WLM performs some prioritization at the address space level rather than the DU level as well.

For example in one CICS region there would most likely be many application programs running at the same time. WLM might do some prioritisation tasks for the entire region, which would affect all DUs within it.

note that multiple DUs within the same address range might be able to execute at the same time, if allowed by WLM and by the available resources.

Types of DUs

TCB

TCBs or Tasks is the type of DU that will be most commonly used, including by application programs

SRB

SRBs or Service Requests are mainly used for system services and tasks executing within an Enclave

Some service requests can be configured to run before tasks, ignoring the dispatching priority rules.