Every SMS system must have an IGDSMSxx member in SYS1.PARMLIB that specifies a required ACDS and COMMDS pair. This ACDS and COMMDS pair is used if the COMMDS of the pair does not point to another COMMDS.

If the COMMDS points to another COMMDS, the referenced COMMDS is used. This referenced COMMDS might contain the name of an ACDS that is different from the one specified in the SYS1.PARMLIB member. If so, the name of the ACDS is obtained from the COMMDS rather than from the IGDSMSxx member in SYS1.PARMLIB to ensure that the system is always running under the most recent ACDS and COMMDS.

If the COMMDS of the ACDS and COMMDS pair refers to another COMMDS during IPL, it means a more recent COMMDS has been used. SMS uses the most recent COMMDS to ensure that you cannot IPL with a down-level configuration.