Prior learning provides the basis for new learning. Mathematics educators employ formative assessment to "elicit" and "use" student thinking as the foundation of their instruction. Yet, information can be elicited and used in a variety of ways, so not all formative assessment is equally "formative." This means that simply eliciting student thinking is insufficient; assessment must impact instruction to be considered formative. Not all methods of eliciting student thinking are equally useful. Ultimately, how teachers access and use student thinking depends on their goals and their specialized knowledge as educators. In this article, the authors propose a framework to better describe nuances in formative practices.