Yay! And it's a little new to me too. The details vary a bit author to author. Sometimes the mark consists of the first words your soulmate will say to you, so some people might have "Hi, what's your name?" and they're like, crap, that's no help at all! Or it could be some other sort of identifying mark, and generally the idea is that the two people involved have to figure it out :)

Glad you liked it!