Questions like this and material like this provide lots of useful information about HTTP POST... but none that I have found clarifies the role of that second URL (or is it a URI, or something else, and is it even secondary?) in the very first line of the POST request header:
POST /second/url/here/ HTTP/1.1
The request itself is sent to a URL (URI?), which feels "primary" to me in some sense. Can someone please clarify the role of both, and why they would be the same or different as seems to be possible?
(P.S. it was probably once so obvious that nobody thought it might ever need explaining. But now when you google for "HTTP POST", half the internet appears, and it's near impossible to see the forest for the trees...)