…a smith mark can be anything from a single letter to several words and lines, inscribed by a wordsmith on an object or creature, in order to modify said object / creature.  There are two basic kinds of smith mark: self-contained, and symbiotic

…One particularly famous kind of symbiotic smith mark is the oath mark.  Oath marks are specially designed smith marks that are the physical representation of some kind of promise, or vow.  They do not enforce the vow – merely serve as a reminder, by glowing and growing warm…

-Excerpt, Ranor’s Structure of Magic