Dylan Wiliam - Formativeness as a potential property of an assessment

I heard "inside the black box" Dylan Wiliam speak at an LSIS event today for the current cohort of Research Development Fellows. Wiliam was fascinating as always. For example here is his definition of formative, which he sees as a property of (some, and only some) assessments:

"An assessment functions formatively to the extent that evidence about student achievement elicited by the assessment is interpreted and used to make decisions about the next steps in instruction that are likely to be better, or better founded, than the decisions that would have been taken in the absence of that evidence."