On https://www.rfc-editor.org/rfc/rfc2326#page-19, it has this section:
4.3 Message Body
See [H4.3]
and many others saying See H[x.y].
What does this H mean? Where can I find this section to read more about? I could not find in the appendix.
It is referring to the HTTP/1/1 specification.
The note is a little bit hidden in the spec but there is section near the top on notation:
2 Notational Conventions
Since many of the definitions and syntax are identical to HTTP/1.1, this specification only points to the section where they are defined rather than copying it. For brevity, [HX.Y] is to be taken to refer to Section X.Y of the current HTTP/1.1 specification (RFC 2068 [2]).
I am guessing you know but just in case - this RFC has been superseded and obsoleted by the later RTSP RFC: https://www.rfc-editor.org/rfc/rfc7826