HCL is a supplementary reference on Examitect's ExAC study plan, not a primary one. That distinction matters for how you allocate study time: the primary references carry the heavier exam load. But supplementary references are on the list for a reason, and HCL is there because the ExAC tests environmental reasoning, not just code compliance.
The six categories where HCL appears are site and environmental analysis, coordinating engineering systems, schematic design, design development, building science and systems, and sustainable design literacy. Examitect's study plan cites specific chapters rather than the whole book, so you can target your reading efficiently by category.
When a question asks you to evaluate a design for passive solar performance, choose an appropriate shading strategy for a given facade orientation, or explain why climate zone affects glazing decisions, HCL is the reference that builds the physical reasoning behind the answer. The primary references may identify the requirement; HCL explains why it works.