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