Answer
Dirty Areas are created when features in a Topology are created, deleted or have their shape edited or subtype value changed. The resulting Dirty Area will not only encompass the extent of the edited features, but will also be expanded slightly outside the envelope of the feature by a factor of 4 times the current cluster tolerance for the topology. The Dirty Area envelope is expanded in order to ensure complete coverage of the edited features when they have been projected on-the-fly.