Marginal notes (also known as head notes) are “the short notations appearing above or beside each section […] of an Act or Regulation” (Sullivan on the Construction of Statutes, 6th ed., §14.59). These notes are intended to help readers identify pertinent provisions in the legislation. The name comes from the fact that they originally appeared in the margins of legislation next to the relevant provisions.
Despite appearing in an act or regulation, marginal notes are not actually part of that legislation. Sullivan is rather disapproving of this:
“Although technically marginal notes are not considered part of legislation, in fact they are physically present and may well constitute the most frequently read component of many Acts and regulations. To ignore whatever light they shed on the meaning of legislation seems artificial and appropriate.” (§14.60)
That said, there are several cases in which marginal notes have been used for legislative interpretation (e.g. R. v. A.D.H., 2013 SCC 28) but this is not uniformly the case. For example, in Imperial Oil Ltd. v. Canada; Inco Ltd. v. Canada, 2006 SCC 46, it says at paragraph 57 “although marginal notes are not entirely devoid of usefulness, their value is limited for a court that must address a serious problem of statutory interpretation.”
Because marginal notes aren’t officially part of legislation, the process of amending them does not necessarily involve an act or regulation. For example, in British Columbia they are amended by the publications staff, not the legislature:
“On this basis, marginal notes are not amended by legislation. They are changed editorially by our publications staff in consultation with legislative counsel. This is done most commonly in conjunction with a legislative amendment to the relevant section, so that the marginal note will better reflect the content of the section.” (A Guide to Legislation and Legislative Process in British Columbia)
This leads to situations in which the only way to know if a marginal note has changed is by looking at the most recent copy of the consolidation produced by that jurisdiction’s Queen’s Printer (or equivalent).