From Isidore of Seville, Etmologiae, book 1, chapter 22:
XXII. DE NOTIS VVLGARIBVS.
 Vulgares notas Ennius primus mille et centum invenit. Notarum usus erat ut, quidquid pro con[ten]tione aut [in] iudiciis diceretur, librarii scriberent conplures simul astantes, divisis inter se partibus, quot quisque verba et quo ordine exciperet. Romae primus Tullius Tiro Ciceronis libertus commentus est notas, sed tantum praepositionum.
 Post eum Vipsanius, Philargius, et Aquila libertus Maecenatis alius alias addiderunt. Deinde Seneca, contractu omnium digestoque et aucto numero, opus efficit in quinque milia. Notae autem dictae eo, quod verba vel syllabas praefixis characteribus notent et ad notitiam legentium revocent; quas qui didicerunt proprie iam notarii appellantur. 
xxii. Common shorthand signs (De notis vulgaribus)
1. Ennius first invented eleven hundred common signs. These signs were used in this way: several scribes standing by together would write down whatever was said in a trial or judgment, with the sections distributed among them so that each scribe would take down a certain number ofwords in turn. In Rome,TulliusTiro, a freedman of Cicero’s, first devised such signs, but only for prepositions.
2. After him, Vipsanius, Philargius, and Aquila, another freedman of Maecenas, added others. Then, after the total number of signs had been collected, set in order, and increased in number, Seneca produced a work with five thousand signs. They are called ‘signs’ (nota) because they would designate (notare) words and syllables by predetermined characters and recall them to the knowledge (notitia) of readers. Those who have learned these signs are properly called stenographers (notarius) today.
Isidore in fact lists various sorts of notae, and some of the manuscripts of the Commentarii Notarium Tironianarum quote him on some or all of them, so it’s worth a quick list:
XXI. DE NOTIS SENTENTIARVM – Critical signs. These are things like asterisks, the obolus, the cryphia, the diple, etc. Things that ancient scribes put in the margins of manuscripts!
XXIII. DE NOTIS IVRIDICIS – Signs used in law. Abbreviations used in ancient law books, like “SC” for senatus consultum, i.e. a decree of the senate.
XXIV. DE NOTIS MILITARIBVS – Military signs. These were symbols placed on the lists or rosters of soldiers, like a T “tau” meaning “alive” or a Θ (theta, for thanatos), indicating that the soldier was killed.
XXV. DE NOTIS LITTERARVM – Epistolary signs. Secret codes used by letter writers to indicate to each other information, while looking innocuous.
XXVI. DE NOTIS DIGITORVM – Finger signals. Gestures of particular meaning.
This work of Isidore seems full of interesting snippets of antiquity. It really needs to be read in paper form – trying to do so from a PDF is frustrating!