Design notes
Reasoning written while these subsystems were being designed — the shape that was proposed, the alternatives weighed, and what the open questions were at the time.
These are historical records, not reference documentation. They describe what was planned, sometimes before it was built and sometimes differently from how it ended up. Where a note disagrees with the reference pages or with the code, the code is right. They are kept because the reasoning is often more useful than the conclusion.
For what katra does today, start with Architecture.