A small annotation such as this one may seem (literally) a marginal detail. However, it may turn out to be an interesting detail, since it sheds some additional light on a hypothesis that has been repeated in scholarship ever since Ceriani published the facsimile edition of the codex in 1876/1883. In the Praefatio of this edition, Ceriani suggestes that the codex was probably not produced for ecclesiastical use, since it includes neither liturgical notes, nor an index of lessons. He notes, though, that the occasional liturgical note occurs in the columns of some texts, but suggests that this is due to the fact that the scribe copied the texts in question from an exemplar that contained such notes (p. 8).Details matter.
More on manuscript Codex Ambrosianus B.21 here and here (cf. here). Cross-file under Syriac Watch.