A sign relation is a formal structure that satisfies a very general definition, on the same order of generality as a mathematical group or geometry. So any consideration of what a particular sign relation contains will be very context-dependent.
We can study sign relations in the abstract or in connection with particular applications. In applications, sign relations describe structures of interpretation, for example, the conduct of sign-using interpreters. Applications divide broadly into descriptive and normative types.
Descriptively, we could be describing the interpretive conduct of someone named “Socrates” who happens to speak English and who uses the word “I” to denote himself. In that case, we would probably want to include the signs “Socrates” and “I” in both the sign domain and the interpretant domain of the sign relation that we use to describe the usage of that agent.