Depends on the role a client can be both front-end component and back-end component. See description in Http semantic and RFC6749 An client is an application (or component) making protected resource (API) requests with, initializing an access_token to request a resource. Clients must be registered at the authorization server (OIDC provider).

In OAuth 2 there is specified mainly two types of clients, public ans confidential. The client type is important when selecting the grant type (how a client can obtain an access token).

RFC679

https://datatracker.ietf.org/doc/html/rfc2616#section-1.3 https://datatracker.ietf.org/doc/html/rfc5849#section-1.1 https://www.rfc-editor.org/rfc/rfc9110.html#name-connections-clients-and-ser