!!! Overview
[{$pagename}] is an __OPTIONAL__ parameter used in the [Access Token Request] defined by [Resource Indicators for OAuth 2.0] and the value of the "resource" parameter indicates a [Resource Servers] where the requested access token will be used.[{$pagename}] value __MUST__ be an absolute URI, as specified by Section 4.3 of [RFC 3986], and __MUST NOT__ include a query or fragment component.  

If the [Authorization Server] fails to parse the provided value or does not consider the [Resource Server] acceptable, the [Authorization Server] __MUST__ reject the request and provide an [OAuth Error] response with the error code "invalid_resource".  

Multiple "resource" parameters may be used to indicate that the issued token is intended to be used at multiple [Resource Servers].

The [{$pagename}] will be sent to the [authorization_endpoint] or the [token_endpoint] depending on the [Grant Type] used.

!! [authorization_endpoint]
When an [Access Token] will be returned from the [authorization_endpoint], and the [{$pagename}] is used in the [Authorization Request] to the [Authorization_endpoint] as defined in Section 4.2.1 of OAuth 2.0 [RFC 6749]. 

!! [token_endpoint]
When the access token is returned from the [token_endpoint], the request parameter is included in the [Access Token Request] to the [token_endpoint]. Sections 4.1.1, 4.3.1, 4.4.2, 4.5 and 6 of OAuth 2.0 [RFC 6749] define requests to the token endpoint with different grant types and others may be defined elsewhere.

!! More Information
There might be more information for this subject on one of the following:
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* [#1] - [Grant Management for OAuth 2.0
|https://darutk.medium.com/grant-management-be9aeff3b394|target='_blank'] - based on information obtained 2022-01-09