In a poly-JES2 environment (that is, an MVS™ system in which more than one JES2 is running concurrently) either one or both JESs can start and pass data sets to a functional subsystem for processing (for example, page-mode printing). A functional subsystem is started by an internally produced MVS START command; JES2 processes the START command so that the functional subsystem started task is associated with the appropriate JES2 (the JES2 that processed the START command). A functional subsystem cannot communicate with multiple subsystems simultaneously.