Every API MUST implement and every API Consumer MUST use the HTTP content negotiation where a representation of a resource is requested.
NOTE: The content negotiation plays the key role in evolving an API, change management and versioning.
A client is programmed to understand the
application/vnd.example.resource+json; version=2 message format semantics. The client requests a representation of the
/greeting resource in desired the media type (including its version) from the server:
GET /greeting HTTP/1.1 Accept: application/vnd.example.resource+json; version=2 ...
The server can provide only a newer version of the requested media type
version=2.1.3. But since the newer version is backward compatible with the requested
version=2 (related: Changes & Versioning) it can satisfy the request and responds:
HTTP/1.1 200 OK Content-Type: application/vnd.example.resource+json; version=2.1.3 ...
NOTE: A server that doesn't have the requested representation media type available MUST respond with the HTTP Status Code 406 Not Acceptable.
NOTE: A server MAY have multiple choices available and MAY respond with the 300 Multiple Choices response. In which case client SHOULD choose from the presented choices.
You can read more about content negotiation at MDN Content negotiation.