In the future of 2019, an Actual is a person who has not been wiped/imprinted, and therefore retains their original personality. This distinguishes them from "butchers" and "dumbshows", who greatly outnumber them, as well as persons like Iris who have been imprinted with the personalities of others for unknown reasons. ("Epitaph One")
Priya Tsetsang, formerly the active Sierra came up with the idea of the birthmark. Jed Whedon and Maurissa Tancharoen (writers of "Epitaph One") suggest that she may have started the Actual's resistance.