DOMSubtreeModified event

The MutationEvents module says this about the DOMSubtreeModified event:

    This is a general event for notification of all changes to the
    document. It can be used instead of the more specific events listed
    below. It may be fired after a single modification to the document
    or, at the implementation's discretion, after multiple changes have
    occurred.  The latter use should generally be used to accomodate
    multiple changes which occur either simultaneously or in rapid
    succession.

I read this to say that an implementation can support the MutationEvents
module even if it never fires anything more specific than
DOMSubtreeModified events.  Furthermore, although it is not recommended,
an implementation may legally choose to fire an event of this type only
after every 100 modifications to the document, or every 30 seconds if
any changes have happened during that interval.

Am I reading this correctly?

Is the intent that when a call to a single DOM method such as
Range.extractContents() causes many mutation events this event is the
one that is fired last, as kind of a summary of all changes that have
occurred? 

       David