What's different between URI, request-URI and URL?

I'm learning Web tech and teacher cannot give me a satisfactory explanation. I'd like to give a few example, please help me to point out am I right.

• I know URL is a subset of URI
• http://www.example.org:56789/a/b/c.txt?t=win&s=chess#para5 is a URL and also a URI
• every address we type in browsers can be called URL
• http://www.example.org:56789/ is also a URI
• the part: a/b/c.txt?t=win&s=chess is request-URI
• the part: para5 does not belong to the request-URI is just a fragment

The URI standard is STD 66, which currently maps to RFC 3986.

URI vs. URL

Section 1.1.3 describes the difference between URIs and URLs (and URNs).

Components

Section 3 describes the components a URI can have.

For the URI http://www.example.org:56789/a/b/c.txt?t=win&s=chess#para5 these would be:

• Scheme: http

• Authority: www.example.org:56789

• User Information: not present

• Host: www.example.org

• Port: 56789

• Path: /a/b/c.txt

• Query: t=win&s=chess

• Fragment: para5

request-URI

The term "request-URI" is not defined or even used in STD 66 / RFC 3986.