The client credentials flow applies when a Client application cannot prompt a human user for input. This flow requires a secret know to the Authorization Service which the Client application can protect. This flow is initiated when the Client sends the client_secret to Authorization Service which validates it and returns an Access Token.
The complete flow is described in RFC 5392, Clause 4.4.
A requestType of “client_credentials” in the UserTokenPolicy (see 6.5.2) means the Authorization Service supports the client credentials flow.