Little need for me to add anything, but I will mention the book “Textual criticism and editorial technique” by (guess who?) M.L. West. It teaches you how to construct a critical apparatus and incidentally how to to use one.
To use an app.crit. properly you need to know about the interrelations of the various manuscripts (MSS), and that’s always discussed in the preface—usually in Latin (but some recent OCT’s use English). But there’s a lot to be gleaned just from the list of manuscript “sigla” that the editor puts just ahead of the text itself (“conspectus siglorum”). Sometimes there’s a “stemma codicum,” which indicates the lines of affiliation among the various MSS (actual or hypothesized!), but this editor doesn’t do that (probably because it can’t be done).
Looking at the “conspectus siglorum” page linked by Qimmik
http://gallica.bnf.fr/ark:/12148/bpt6k25361m/f9.image
we see that it’s more than a simple list of MSS and their sigla (F etc.).
The editor organizes the medieval MSS into two “families”—the F family, which the editor designates f, and the C family, designated c. So when we see these lower-case letters in the apparatus it means they’re referring not to a single MS but to a whole group.
And then there’s the even more important L, representing the “consensus codicum”, i.e. all the MSS (or perhaps all the MSS with the exception of whatever individual MSS are cited). It invites misunderstanding to use an apparently random upper-case letter to represent not a single manuscript but the whole lot of them, but there it is, editor’s choice.
And in addition to the medieval MSS there are a couple of fragmentary manuscripts on papyrus too (therefore older than the med.MSS), jointly designated O (for Oxyrhynchus, where they were both found). In his preface (sorry, praefatio!, it‘s all in Latin), the editor notes that since these older manuscripts don’t agree consistently with either one or the other “family” (f and c), the split between the two families will have come at a later point in the transmission. And occasionally papyri will offer readings that do not survive in the medieval tradition at all. But there’s papyrus evidence for only small portions of the text. (How different would the text look if there were more, one may wonder.)
So: looking now at the first page of the text, linked by kingbenlucas,
http://gallica.bnf.fr/ark:/12148/bpt6k25361m/f10.image
we see quite a number of entries for line 1, that is the title.
One manuscript (M) adds ῥήτορος to Xenophon’s name. The rest, we are to infer, do not.
Then we have “ἀναβάσεως L”. In his title the editor prints not the genitive—the reading of all the MSS, as the “L” informs us—but the nominative Ἀνάβασις. Apparently he’s done that on his own initiative, since he cites no manuscript for it. In ancient Greek practice the title would run Ξενοφωντος (or Ξενοφωντος ρητορος) Αναβασεως α’, “bk. 1 of the Anabasis of Xenophon.” The editor has chosen to change that. (Why?, you may well ask,)
For “Α” itself (“bk. 1”), various MSS give various expansions (λογος πρωτος etc.), and only F has the simple α’ (α with a bar-line above to show it’s a numeral).
And so it goes on. I pick out a couple of later entries on this page.
In line 3 (the 2nd line of the text proper) only M has the spelling Ἀρτοξέρξης, preferred by the editor. The rest—“rell.” for reliqui (sc. codices)—all have Ἀρταξέρξης. Here the editor adds “(it. infra)", i.e. “item infra”, “likewise below.” This indicates that the editor won’t mention this spelling variant in future: whenever we find Ἀρτοξ- in the text, we are to assume that all the MSS except M have the spelling Ἀρταξ-. The editor will print Ἀρτοξ- everywhere, but only one manuscript spells it in this form.
In line 5 we have οἱ τὼ reported for Aristides. Aristides quotes this passage (we’re not told how much he quotes, but we can look it up if we want to know), and instead of εβουλετο τω παιδε etc. he has εβουλετο οἱ τω παιδε etc. The editor includes the τὼ so that we can tell just where the οἱ comes. (Alternatively he could have recorded ἐβούλετό οἱ, but that would have taken up more space.) The οἱ will be the dative of the reflexive pronoun, and it could well be right. The “indirect tradition” of Xenophon and of many other authors often gives readings that don’t survive in the direct tradition (i.e. in the MSS of the text), and quite often papyri will have the same reading (our Anab. editor notes this In his preface as being true for the Anab.). So here’s a real textual question: does the οἱ belong in Xenophon’s text or not? This editor judges not. Others might judge otherwise.
In judging between variants such as these a good critical principle to apply is encapsulated in the question “utrum in alterum?”, in full “utrum in alterum abiturum est?” “Which of the two readings would have been changed to the other?” On the assumption (not always correct) that one of the readings is original and the other Is a corruption of it, which is more likely to be the corrupted one? ((In this last instance, for example, is οἱ more likely to have been added to the original text, or is it more likely to have dropped out?)
So to use the app.crit. is to engage with textual issues: did the author in fact write this, or something else? There’s often a temptation to take issue with the editor’s decisions, but we have to bear in mind that the editor will have studied the text and its manuscripts much more thoroughly than we have, so we should always be hesitant of disagreeing, especially when the editor is as experienced and well-respected as this one is. But that needn’t stop us exercising our own critical powers. That’s what the app.crit. is for.
I didn’t mean to write so much!