Kevin P. Fleming recently posted an entry on the Asterisk Wiki about Asterisk SCF that goes as follows:

“… the Asterisk SCF SIP components will provide all the mechanisms required to implement authentication of incoming requests, but they won’t provide anything but the most basic mechanisms to decide whether authentication of a particular request should be required. These basic mechanisms will suffice for very simple networks, but they won’t support dynamic or complex decision making.

… an Asterisk SCF system administrator or application developer can deploy a small ‘hook’ component, one which will receive notification that a SIP request has arrived, along with all its gory details, and it can then tell the requesting SIP component whether that request should be accepted as-is, or if authentication should be requested, or if the component defers to make a decision (allowing another, lower priority, component the chance to decide).”

More information can be obtained here