Registration process by OAuth2

I have been analyzing security process in 3gpp specs and the registration process in the spec. explained with that flow. The flow represents the registration of a client to the server.From SPEC. spec 2

I am not a security specialist and i do not understand why we send Oauth2 token in step 1. Doesn’t the access token send after the registration? I know that the access token used for authorization if the client known. I know that is a weird topic and I tried to explain it briefly i hope you understand if you dont you can always ask me.