The multi-stream feature internally breaks a single job into multiple pieces as streams. This results in multiple sub-jobs, where each stream performs a subset of the work defined in your job. This improves job parallelism, which can increase job performance.
Multi-stream is only available when using a tape backup product. When using the associated GUI to interface with a tape backup product, you can specify how many streams to use for your job. However, if you run ARC from the command line, there is no way to specify multistream so you will always run as a single-stream job.
The multi-stream feature is not supported on Mainframe systems.