The term rubric from the Norman French rubrique simply means something written in red. Of course the way rubric is used today does not literally mean it is in red. This can apply to lots of things, headings, important notes to the text, feedback comments etc.
The OED list of definitions, with the examples missing runs as follows:
A. n. I. Something traditionally written in red, and related uses.
Cf. red letter n.
1.a. A direction in a liturgical book as to how a church service should be conducted, traditionally written or printed in red ink. Also
b. An established custom; a set of rules, an injunction; a general
†c. The rule of a religious order. Obs. rare—1.
d. An explanatory or prescriptive note introducing an examination
2.a. A heading of a chapter or other section in a book or manuscript, written or printed in red, or otherwise distinguished in lettering; a
particular passage or sentence marked in this way. Also in extended
b. The heading of a statute or section of a legal code.
c. fig. A descriptive heading; a designation, a category.
†3. A calendar of saints; an entry in red letters of a name in such
a calendar. Also in extended use. Obs. (rare after 17th cent.).
In Spanish- and Portuguese-speaking contexts: a decorative flourish attached to a signature; (also) a mark used in place of a
signature. Now chiefly hist.
II. A substance used for marking in red.
†5. Red ochre, ruddle. Obs.
†6. A preparation for reddening the complexion. Obs. rare—1.
a. Of lettering: written or printed in red. Also fig. The example in
quot. ?c1475 may represent the noun.
b. Designating a pillar or post inscribed with the titles of books for sale. Now hist.
a. Chiefly poet. Red, ruddy. Now rare. In quot. 1921 with punning
allusion to sense B. 1a.
†b. Designating certain lake pigments. Obs.