Add 3-legged access token definition

documentation/CAMARA-API-access-and-user-consent.md

@@ -61,6 +61,8 @@ The list below introduces several key concepts:
 - `Operator`: Mobile Network Operator (MNO), or CSP/telco operator, exposing network capabilities via standard CAMARA APIs.
 - `Aggregator`: aggregate Operator’s CAMARA standardised APIs for building services offered to application developers. An aggregator can be a hyperscaler (e.g. Vonage, AWS, Azure, Google Cloud) offering its own services or directly exposing CAMARA APIs available at the operators, or it can be a telco operator acting as an aggregator, i.e.: aggregating other telco operators and exposing CAMARA APIs available at these telco operators.
 - `API Exposure Platform`: Operator's platform for exposing network capabilities via standard CAMARA APIs. It is the platform that exposes the CAMARA APIs to application developers and provides the authentication and authorization mechanisms to access them. It is also responsible for consent management. It typically consists of at least an Auth Server and an API Gateway.
+- `3-legged access token`: Access tokens are created by the authorization server to be used by the client at the resource server. If the authorization server authenticates the user and potentially asks for their consent for the API access, then the acccess token is called a 3-legged access token, because of the three involved parties: user (the resource owner), authorization server (operator, i.e. service provider), and the client (third-party application). Typically 3-legged access tokens are created in CAMARA through OIDC Authorization Code flow or CIBA.
+- `2-legged access token`: Unlike the 3-legged access token, which involves user interaction, the 2-legged access token involves only the client and the authorization server, not the user. It is a server-to-server communication, and the authorization neither authenticates the user nor is the user asked for their consent.
 
 >[TO BE EDITED/COMPLETED]