SYSTEM is a valid PARM under CA Easytrieve Plus release 6.4 0311. It is not valid for CA Easytrieve release 11.6.
SYSTEM parameter under release 6.4 is documented in the Reference Guide with this information:
Specifies the host operating system under which the system will be running.
Valid options are DOSVSE or OS. DOSVSE refers to version AF2 or higher.
Release 11.6 only supports the OS system, so this parameter is not necessary or relevant under release 11.6.