This is related to the “Modified file” (FileModified) activity. A FileModifiedExtended event is logged when the same person continually modifies a file for an extended period (up to 3 hours). The purpose of logging FileModifiedExtended events is to reduce the number of FileModified events that are logged when a file is continually modified. This helps reduce the noise of multiple FileModified records for what is essentially the same user activity, and lets you focus on the initial (and more important) FileModified event.