If you have multiple HSMplexes in a sysplex, you must use the SETSYS keyword PLEXNAME in a ARCCMDxx member of SYS1.PARMLIB.

SETSYS PLEXNAME(HSMplex_name_suffix)

The PLEXNAME keyword distinguishes the separate HSMplexes within a single sysplex. If you have only one HSMplex in a sysplex, you can use the default name. The default name is ARCPLEX0: the suffix is PLEX0, with a prefix of ARC.