A new grant type for a token exchange request and the associated specific parameters for such a request to the token endpoint are defined by this specification.
A token exchange response is a normal OAuth 2.0 response from the token endpoint with a few additional parameters defined herein to provide information to the client.
It is true that some members of the identity system might have awareness that impersonation is going on, but it is not a requirement.
Web Service clients have used WS-Trust as the protocol to interact with an STS for token exchange, however WS-Trust is a fairly heavyweight protocol, which uses XML, SOAP, etc.
Whereas, the trend in modern Web development has been towards lightweight services utilizing RESTful patterns and JSON.
Delegation semantics are different than impersonation semantics, though the two are closely related.
With delegation semantics, principal A still has its own identity separate from B and it is explicitly understood that while B may have delegated some of its rights to A, any actions taken are being taken by A representing B. Delegation and impersonation are not inclusive of all situations.
The OAuth 2.0 Authorization Framework have emerged as popular standards for authorizing and securing access to HTTP and RESTful resources but do not provide everything necessary to facilitate token exchange interactions.