The fields you can assign depend on the event type - there's about a dozen that ALL events have (the ones up to ExtraneousInfo if you're looking in the console), then stuff like source/destination IPs depend on which event ends up firing your rule and being used in the email.
The event types are described in the manual, but I don't see anywhere the fields are. If you open something like the filter or rule builder, you can click through the events and see the fields they include to get an idea.