I’m reading through the first volume of Diogenes Laertius Lives of the Philosophers. In book 3, devoted to Plato, we find the following interesting excursus, which I copy from a version present on Wikisource here.
65. The right interpretation of his dialogues includes three things: first, the meaning of every statement must be explained; next, its purpose, whether it is made for a primary reason or by way of illustration, and whether to establish his own doctrines or to refute his interlocutor; in the third place it remains to examine its truth.
And since certain critical marks are affixed to his works let us now say a word about these. The cross × is taken to indicate peculiar expressions and figures of speech, and generally any idiom of Platonic usage; the diple (>) calls attention to doctrines and opinions characteristic of Plato; 66. the dotted cross (⨰) denotes select passages and beauties of style; the dotted diple (⋗) editors’ corrections of the text; the dotted obelus (÷) passages suspected without reason; the dotted antisigma (Ꜿ) repetitions and proposals for transpositions; the ceraunium the philosophical school; the asterisk (∗) an agreement of doctrine; the obelus (−) a spurious passage.
So much for the critical marks and his writings in general. As Antigonus of Carystus says in his Life of Zeno, when the writings were first edited with critical marks, their possessors charged a certain fee to anyone who wished to consult them.
65. A wedge-shaped mark >, used in early papyri to denote a fresh paragraph.
It is always good to see the actual basis for some of the remarks that get made in text critical handbooks. Here at least, we have an explicit statement of what marks indicate what.