Educational theories offer teachers useful conceptual tools for developing teaching. However, such theoretical concepts are often hard to learn, and to teach. Phenomenography and Variation Theory (PVT), and especially the concept critical aspect, is an example of a powerful tool for teachers when designing teaching. When teaching this concept, educators could benefit from knowing what learners need to discern to understand and use the notion of critical aspects. In this study, this is explored by using PVT to analyse written material from teachers undergoing an introduction to Variation Theory. The study identifies five qualitatively different conceptions of the notion of critical aspects. By comparing the differences between these conceptions, four aspects were identified that address what the teachers need to discern in order to perceive the notion of "critical aspects" in the targeted way. The results have potential for developing in-service training and teacher education on theoretical concepts.