This study examines prospective secondary mathematics teachers’ understanding and sense making of representational quantities generated by algebra tiles, the quantitative units inherent in the nature of these quantities, and the quantitative addition and multiplication operations—referent preserving versus referent transforming compositions—acting on these quantities. Two student–teachers constantly relied on an additive interpretation of the context, whereas three others were able to distinguish between and when to rely on an additive or a multiplicative interpretation of the context. The results indicate that the identification and coordination of the representational quantities and their units at different categories are critical aspects of quantitative reasoning and need to be emphasized in the teaching–learning process.