Martin (1992: 528-9):
Whereas status addresses the concerns of social hierarchy, contact is concerned with the degree of involvement among interlocutors. This is determined by the nature of the fields speaker/listeners are participating in — how much contact they involve, how regularly, whether work or leisure activities and so on. Poynton's (1985/9: 77-78) field oriented contact distinctions have been reworked slightly here; the notion of contact appears to be equivalent to what Hasan (1977: 231-2, 1985/9: 57) refers to as social distance, which for her is determined by "the frequency and range of previous interaction" (1977: 231).
Contact is not determined by field. In logical terms, the relation between tenor and field systems is extension — they are conjunct systems — not enhancement (cause-condition). Less abstractly, a given field may involve interlocutors who have had many previous interactions, some previous interactions, or no previous interactions at all.