Each network event is represented by a so-called "Event Record" (sometime also named an "InfoUnit", an "Unit of Information"). Data obtained from all services will end up as an event. For example, Windows Event Log data, syslog data and a file line obtained by the file monitor will all be an event. That kind of generalization make it easy to deal with all of these events in a consistent way.
Each event has a set of properties which in turn have values. For example, there is a property named "source" and it will always contain an indication of which system the event orginated on. Obviously, not every event source does support all properties. For example, a syslog message does not contain a Windows Windows Event ID - simply because there is no such thing as an event ID in syslog. So, depending on the type of event, it may contain different properties.
In order to make the product really generally useful, some few properties have been defined in a generic way and are guarantted to be present in every event, no matter what type it may have. Sometimes this is a "natural" common property, like the "fromhost". Sometimes, though, it may look a bit artificial. An example of the later is the "syslogfacility" property. It is guaranteed to be present in every event - but actually this is a syslog-only thing. The non-syslog event sources either emulate this property (in a consistent manner) or allow the user to configure a syslogfacility that should be used for all events generated by that service. At the bottom line, this will ensure that the property is available in all events and - given proper configuration - that can be extremely helpful for the administrators to set up things in a powerful and generic way.