This event occurs whenever a persistent object is modified by setting one of the generated properties. It also occurs when the object is restored to its original value by RestoreUnmodified, or is refreshed from the database using Refresh, RefreshAndLock or RefreshTree, or by a database rollback.

If you look at code generated for properties, you can see the OnModified method being called, which raises this event. The event also occurs whenever an object adopts or associates a member, or whenever an object is adopted, since it's parent changes at that point.