The SPLEVEL parameter is specified in the PANSYSxx member of SYS1.PARMLIB.
It is a required parameter and must be five characters long.
It specifies the version, release, and modification level of the z/OS operating system that the subsystem is running on.
For example: The SPLEVEL(s) for z/OS are as follows:
z/OS Version 2.1 SPLEVEL=7.2.1
z/OS Version 2.2 SPLEVEL=7.2.2
z/OS Version 2.3 SPLEVEL=7.2.3
z/OS Version 2.4 SPLEVEL=7.2.4
z/OS Version 2.5 SPLEVEL=7.2.5
It only influences the subsystem.
On the System console, enter the command - D IPLINFO
The returned display contains the Release information including the SPLevel:
IEE254I hh.mm.ss IPLINFO DISPLAY nnn
SYSTEM IPLED AT hh.mm.ss ON mm/dd/yyyy
RELEASE z/OS 02.05.00 LICENSE = z/OS <== SPLEVEL=7.2.5
It will work for z/OS releases without changing it every time.
It was really only used in the Product back in the MVS/XA, MVS/ESA days.
For z/OS 2.5 can specify 7.2.5 or can keep what was specified prior. (MINIMUM is 7.n.n)