A Declare OnChange rule automatically executes an activity, or suspends the current work object, when the value of a specified property changes.These are fired purely based on changes to the clipboard.
This makes them especially useful in pure business rules engine applications which often cannot rely on persistence events
As an example, these OnChange rules help drive a unique BPM feature, Policy Overrides. Policy Overrides allow for the dynamic and declarative override of a flow, based on changes to the data on the clipboard.
This is covered in more detail along with a good example in policy override section of CASE MANAGEMENT courses in pega academy.You can go thru them to learn more about it.