Well – this is not completely new, but we redesigned it a bit.
Extension grants are used to add support for non-standard token issuance scenarios to the token endpoint, e.g. translating between token types, delegation, federation, custom input or output parameters.
One of the common questions we got was how to implement identity delegation – instead of repeating myself here – I wrote proper documentation on the topic, and how to use IdentityServer4 to implement it.
Get the details here.