Source properties are defined by a particular dispatcher implementation. For example, the inbound and outbound protocol event dispatcher uses a "Rule" and "Priority" property to discriminate when a sink is to be notified of an event. Most transport event sinks do not use the "Rule" source properties, except the OnTransportSubmission event. All protocol and transport events support use of the "Priority" source property. (Nov. 2001 PSDK).
Basically this means that you can use the "Rule" to specify under what conditions the registered sink should fire.
The following example is taken from the SMTP Protocol Security Ehlo sink.
Sink properties (not illustrated as its the same format as for Source properties) are reserved for a particular sink implementation. It is the responsibility of the sink developer to specify what Sink properties, if any, to use.
Incorrect use of the e-nspect Event Sink Administration Module can seriously damage you SMTP installation.