Broadly speaking in the context of the document:
- A "box" is an element, with a width/height.
- A "content rect" is the size of that "box", but subtracting border/scrollbar/padding which might be on that element.
- A "overflow rect" is the size of how large the scrollable area is (if an element has "overflow: scroll" on it). E.g. how much the element can scroll in any particular direction.
The calculation of the "overflow rect" calculation occurs in this class is: third_party/blink/renderer/core/layout/ng/ng_layout_overflow_calculator.cc