‘Pointer to Optional Part ‘ in ISUP

As we know that in any ISUP Message’s Mandatory Variable Part we have a paremeter called “Pointer to start of Optional Part “. However in a ISUP message like CPG,ACM we have only Manadtory Fiaxed Part and then Optional Part only . Now in such messages does the “pointer to Start of ptional Part ” still there or we have straightaway the “Optional Parameter= X” field. Please clarify