Halliday & Matthiessen (1999: 376-7):
The interaction base provides the resources for enacting social rôles and relations as meaning, (prototypically) in dialogue. The interaction base includes the strategies for adopting and assigning speech rôles, for giving and demanding assessments, and the like. The locus of these strategies is a unit of interaction or move. A move is typically mapped onto a figure from the ideation base: a speaker construes a quantum of experience as a figure and enacts this figure as a move in dialogue, either as a proposition or as a proposal. This mapping between figure and move is a central feature of the way we jointly construct and negotiate experience.