Richard,
I cannot give you the PMBOK answer to this, but I can provide some guidance:
A "social contract" is a IMPLICIT agreement among the team (party, management, etc.) to behave in a certain way. In other words, "professionalism." For example we do not comment on one's weight, religion, gender, etc. so that we can all function towards the same goal.
A "team contract" is (and this is my opinion) a set of "rules of the road" for team behavior and they are EXPLICITLY stated. For teams that I am leading, there is a set of rules and they are written. For example, "During virtual meetings, use the "raise hand" function and mute your microphone" and behavioral rules, including "treat each other professionally."
Hope that helps a little.
Harry