I know this post is from forever ago, but this is actually something I still have to explain to people currently, so
The tattoo that many of the Scions and several of the Sharlayan NPCs have in conspicuous places mark them as Archons, sages of their disciplines. This makes them easily identifiable when they want, among other things, to access the restricted sections of Sharlayan libraries. It's the equivalent of tattooing your Ph.D. somewhere visible so the other academics know you belong among them. This is why we likely won't get this as a tattoo option ever. Though I'm still hoping a certain character will some day be able to make an Archon thesis out of
(5.x spoiler)her studies of tempering and how to treat it