This is a property of type Page. It represents a person who put an original signature, initials, mark, or seal on a manuscript text. It should only be used if the manuscript contains original signs. If the manuscript is a copy that does not contain the original signs, the value for this property should be None. If a later copy has a signed certificate of authenticity written on the same paper, the certificate should be represented as a separate child text. If the original of the copy does not survive, you can create a Lost manuscript text and enter the signatories there.
We do not distinguish between signatures, initials, and marks because the boundaries between these things can be hard to define, and because Signs of Literacy is already collecting data about literacy.