Clients In OAuth2 and OpenID Connect
OAuth2 and OpenID Connect both have the notion of a client. In this area, a client is any application or service that relies on the authorization server for authorization.
Clients have their own credentials. They have a client identifier and a secret that they provide to the authorization server to, well, provide proof that they are authorized to obtain or verify tokens on behalf of a user. JWT carry the client id as a way to determine which client requested the token and whether they are known to the client receiving said token, and they may be listed as the audience of a token.
An interesting authorization flow is the client credentials flow. This allow a client to obtain a token to identify itself to other services. This is different than the usual use case for OAuth2 based authorization where a service or an application acts on behalf of a user. In this case, the token is to show that the client wishes to identify and show authorization to other clients with a common authorization service.
This has some interesting consequences in an environment where services are no longer contained and protected by a common firewall. It shifts the authorization model from “you can reach me, therefore you are trustworthy” to “we trust a common third party, therefore we will trust each other” model.