The MMCDA Document is a legal document which has specifications adopted by municipalities and incorporated into their bylaws. For this reason, all supplementaries are held separate from the original document itself so that in referencing the specifications, if a municipality or district has not approved the supplementaries, the original specifications are not "missing" from the document.
On the MMCD website you will see the menu heading "Resources". If you expand this selection you will see "Supplementary Updates" in the list of options. Supplementary updates for all editions are listed on this page.