This study aims to pedagogically validate CEFR-aligned science descriptors, in particular, contributing towards physics literacy descriptors. It explores how disciplinary literacies align with the pedagogical talk and actions of a CLIL Physics teacher, drawing on Dalton-Puffer’s construct of Cognitive Discourse Functions (CDFs). CDFs have proven useful for identifying subject-specific language demands in L2 content subjects and serve as the foundation for a heuristic assessment tool for CEFR-disciplinary literacy descriptors. Using classroom interaction data, the study examines the applicability of CEFR-aligned disciplinary literacy descriptors – originally developed for assessing students’ writing – to teacher talk in whole-class teaching episodes. The study pursues two aims: first, to provide qualitative insights into how the CDFs Define, Describe and Explain are embedded in pedagogical meaning-making in a seventh-grade bilingual CLIL Physics lesson, using two complementary analytical approaches; and second, to explore how these insights can inform and refine the heuristic tool for CEFR-aligned science literacy descriptors. The findings contribute to the pedagogical validation and further development of the heuristic assessment tool, strengthening its relevance and applicability for classroom interaction in secondary CLIL Physics education.