!!! Overview
[{$pagename}] [HTTP Request] [HTTP Header Field] defined in the [Token Binding over HTTP] 

Once a client and server have negotiated the Token Binding Protocol with HTTP/1.1 or HTTP/2 (see The [Token Binding Protocol] and [Token Binding Protocol Negotiation|Transport Layer Security (TLS) Extension for Token Binding Protocol Negotiation]), clients [MUST] include a [{$pagename}] [header field|HTTP Header Field] in their [HTTP Requests], and [MUST] include only one such header field per [HTTP Request].  Also, The [{$pagename}] field [MUST NOT] be included in [HTTP Responses].  

The ABNF of the  Sec-Token-Binding header field is (in [RFC 7230] style, see also [RFC 7231] Section 8.3):
%%prettify 
{{{
Sec-Token-Binding = EncodedTokenBindingMessage
}}} /%
     
The header field name is "[{$pagename}]" and its [SINGLE-VALUE], EncodedTokenBindingMessage, is a [base64]url encoding of a single TokenBindingMessage, as defined in [The Token Binding Protocol|Token Binding Protocol], using the [URL]- and filename-safe character set described in Section 5 of [RFC 4648], with all trailing pad characters '=' omitted and without the inclusion of any line breaks, whitespace, or other additional characters.

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]