In my interpretítion one of the reason why it is hard to elaborate a clear definition for formative assessment that is to compere two different aspects of assessment in the discourse. Sumative refers to a function of assessment. We can realize summative assessment on the ways how learners learn closing a developmental period for example (ie summative assessment in formative assessment).
Formative assessment refers to a dimension of learning and assessment. Shortly, on how to improve learners' own learning activites, competences achieving higher level understandig with also measurable imporvement in tested achievement.
So the classic dichotomy of summative and formative assessment - trough the work of the discourse from Scriven, Bloom, Sadler, Black-Wiliam, ARG, Benett etc - developed the concept so that it became a simple logical argumentation fallacy. Compering a function to a dimension in the definition, based the concepts as two parts of the same level dichotomy.