Section on "frame is cross origin"

[garykac]

"there's like 4 different ways of saying that a frame is cross origin in specs and I can never wrap my head around them. Might be nice to have a section there as it comes up in many specs. "

Relevant spec: https://html.spec.whatwg.org/multipage/browsers.html
In particular, sections 7.1 and 7.2. Among the notable terminology: "unit of related browsing contexts" and "IsPlatformObjectSameOrigin(O)"

See whatwg/html#2112
but it's really unclear in the end what the use cases are. If someone could assemble them that would be helpful. In general what I'm unsure of is what the X and Y are in "X is cross-origin with Y". Sometimes it's frames, sometimes it's platform objects, sometimes it's JavaScript code, sometimes it's network resources...