Kilos, km etc are concepts, not percepts or sensory data: They are mental units with existential connection & dependence on reality. Most philosophical traditions disconnect concepts from reality in the Platonic tradition. My recommendation to read Rand's Intro to objectivist epistemology was sincere.
I believe that this was the first theory of concepts that successfully established their links and dependence on existence and reality. She covers axioms, simple concepts of objects, concepts of relationships and derivative, second and third order concepts. It was challenging for me to grasp in the 1970s, but still seems consistent and logical 50 years later.
The second edition has a paper by Dr Leonard Peikoff on the Analytic-Synthetic Dichotomy which is brilliant. There is also a sizable section where Rand answers common technical questions regarding her theory of concepts.