From The Heretic Knowledge Vault
Travel runes serve about the same function in warp gates that e-tickets do in real-world airports: they allow the bearer to get from the gate to a specified place. We don't know exactly how they work, i.e., whether they supply any of the required magic themselves, but they're classified as magical gadgets just in case.
We've seen them used three times. Meji has to get one to travel from Tsuirakushiti to Saus as she starts her research project. Later, she, Sarine and Jon buy runes to get from Taberna to Port Lorrel. Finally, Ian needs one to get from Saus to Tsuirakushiti, but because one must show identification when buying a rune, he's reluctant to get it conventionally, probably for fear of being recognized as the person who started the firestorm (with a little help from Toren, to be sure). He solves this problem by liberating one from an old man who's getting out of Saus. We learn as a by-the-by from this that travel runes aren't coded for the purchaser, a security deficiency that the Powers That Be should probably remedy. (Maybe the better analogy is to subway tokens rather than airline e-tickets; subway tokens don't care who uses them either.) Presumably Jon, Sarine and Sara needed travel runes to gate into Tsuirakushiti, but we didn't see the transaction.