The explanatory note about the kind of information a source contains is called a(n)

It is called a citation which is also called as reference. A citation or reference identifies a source of information (uniquely). It is an article which usually has two parts: (1) each section of text is marked as an inline citation which is either based on or quoted from an outside source. the said inline citation may be a superscript footnote number or a short citation; and (2) list of full references which gives complete and formatted detail about the source which enables readers of the article can find it and verify it.