Implicit means it is recoverable from the discourse... i.e. we recognise an imperative as a move in an action exchange, likewise for your indicative examples below… moves in a knowledge exchange.
Here Rose uses 'discourse' to mean 'discourse semantics' as the (more abstract) stratum above lexicogrammar on Martin's (1992) model (eg 'moves in action and knowledge exchanges').
But note that what he is actually referring to is the cotext, the accompanying text, and this can be viewed at different levels of symbolic abstraction: as wording (lexicogrammar) or meaning (semantics). This again evinces the same confusion of two distinct dimensions, cotext with symbolic abstraction, inherent in Martin's stratification, as argued in previous posts.