This is part of the Netscape 4 event management suite which allows events to be routed to handlers other than just the one that defaults to being associated with an event.
The events to be captured are signified by setting bits in a mask.
This method provides a means of indicating which events are no longer needing to be captured by the receiving Document object.
The events are specified by using the bitwise OR operator to combine the required event mask constants into a mask that defines the events you want to capture. Refer to the Event Type Constants topic for a list of the event mask values.
Since this is only supported by Netscape prior to version 6.0, the functionality is likely to be deprecated when the standards bodies agree on a standard way of handling events. In the meantime, we shall have to implement scripts using this capability if we need to build complex event handling systems that work on legacy browsers. A different script will be required for MSIE although it may be possible for Netscape 6.0 to share the same one.
You may be able to factor your event handler so that you only have to make platform specific event dispatchers, and can call common handling routines that can be shared between MSIE and Netscape .