  There are no sharp borderlines between commentaries proper, textbooks, encyclopedias, and treatises. (Philipp Melanchthon’s works on ethics might be a good illustration of this: “Melanchthon on Ethics” starts as something which is a commentary on parts of the Nicomachean Ethics [Melanchthon, 1535] and—in the 1550 and later versions, now edited and translated by Günter Frank—was  developed into something which is a textbook in which rather few traces of and similarities to the commentary genre are left [Melanchthon, 2008].)  And even where the work in question is a commentary in the most narrow sense of the word, the text can be some sort of bibliographie raisonée of the previous literature (e.g., some of Augustinus Niphus’s texts), a decent guide to some probable meaning of Aristotle’s text (e.g., some of Cesare Cremonini’s commentaries on the Parva naturalia), or a text written in order to influence the political world of the day (e.g., Antonio Montecatini’s commentary on the third book of Aristotle’s politics [Montecatini, 1597]), or anything in-between.