From CA Datacom/DB System and Administration Guide:
(Optional) Specifies the database ID (the DATACOM-ID attribute-value of the database definition in Datadictionary) which contains any dynamic system tables used to obtain real-time information as the MUF is running.
Note: You must specify the SYSTEMDBID MUF startup option in order to activate Dynamic System Table access from the MUF.
If you do not specify the SYSTEMDBID option, attempts to access any of the dynamic tables receives a no record found condition.
The database specified in SYSTEMDBID is never opened to DASD and should not be defined as virtual. <===