Summary of PLEXCFG and GRS system parameters

If you do not have an existing global resource serialization complex:

Use GRS=NONE and PLEXCFG=XCFLOCAL or MONOPLEX when you want a single-system sysplex and do not want a global resource serialization complex.
Use GRS=TRYJOIN (or GRS=START or JOIN) and PLEXCFG=MULTISYSTEM when you want a sysplex of two or more systems and want a global resource serialization ring complex that matches the sysplex and uses XCF services.
Use GRS=STAR and PLEXCFG=MULTISYSTEM when you want a global resource serialization star complex.

If you have an existing global resource serialization complex:

Use GRS=START or JOIN and PLEXCFG=XCFLOCAL or MONOPLEX when you want a single-system sysplex and want to maintain an existing global resource serialization complex that uses non-XCF protocols.
Use GRS=START or JOIN and PLEXCFG=MULTISYSTEM when you want a sysplex of two or more systems and want to migrate systems in an existing global resource serialization complex. Systems that are in both the sysplex and complex use XCF services; systems that are not in the sysplex but are in the complex, use non-XCF protocols.