You know of jobs, spooled files, and so forth, by their names. The system uses an ID that is associated with the name. The ID is assigned based on usage.
Several of the APIs either require or allow you to use an internal ID. When you use an internal ID, it is generally faster because the system does not have to convert the external name to the internal ID.
A variety of terminology is used to identify internal IDs. For example:
The internal values are often accessed in one API and then used in another. For example, if you want a list of jobs, you would use the List Jobs (QUSLJOB) API, which provides the internal job ID for each job in the list. You could then use the internal job ID to access a spooled file for a job with the Retrieve Spooled File Attributes (QUSRSPLA) API.