Martin (1992: 531-2):
Contraction is easiest to illustrate from phonology, where various reduction processes make the casual conversation of intimate friends and family almost unintelligible to outsiders. Proliferation is easier to illustrate at the level of discourse semantics, where the choice of subject matter for example expands considerably the better more people get to know each other.
This misrepresents subject matter (field) as discourse semantics. The confusion is thus along the dimension of stratification (context vs semantics). In terms of stratification, this confusion is in the opposite direction of Martin's more common misrepresentation of activity sequences (semantics) as field (context).