The Initial Filter Criteria is the grouping between a Trigger Point (the logical expression matching a message) and an Application Server. The absence of a Trigger Point in an Initial Filter Criteria indicates that the message should always be forwarded to the respective Application Server.

The Initial Filter Criteria can only match Initial Requests, like INVITE, REGISTER, SUBSCRIBE, MESSAGE, etc. Subsequent messages (like BYE, NOTIFY, INVITE inside a dialog, etc) are not checked or forwarded.

Each Initial Filter Criteria can be set to match only when the respective user on which it is set is registered or unregistered or in both cases.