As models of how students' thinking may change over time, learning progressions (LPs) have been considered as supports for teachers' classroom assessment practices. However, like all models, LPs provide simplified representations of complex phenomena. One key simplification is the characterization of student thinking using levels—that is, the twin assumptions that student thinking is both coherent and consistent. While useful for the design of standards and curricula, the LP level simplification may threaten the basic premise that LPs could be used to diagnose a student's level and then provide tailored instruction in response. At the same time, our work with teachers suggests that, even with their simplifications, LPs may be useful in the classroom. Thus, rather than abandoning LPs, we sought to understand their potential affordances by exploring how teachers learn from LPs (knowledge-for-practice) and contribute to deeper understanding of LP use (knowledge-of-practice) as they identify and enact uses of these tools. To do so, we engaged high school physics teachers in a 2-year, LP-based professional development program. Based on qualitative analyses of planning meetings and interviews with the teachers, we describe how teachers used LPs to support classroom assessment with varying reliance on the LP level simplification. Although teachers used LPs in ways that relied on the coherence and consistency assumptions of the LP level simplification, uses of LPs that did not require these assumptions were more prevalent both within and across teachers. This study's findings have implications for research, teacher professional development, and the design of LPs.