or
When a field on your layout has a rule that references it, and that the user changes the value of that field, then an onchange rule is triggered. For example, you may have two fields on a layout, named amount1 and amount2. You may have an onchange rule:
total = amount1 + amount2;
When the user changes either amount1 or amount2, the total field on the layout will be updated via an Ajax call to the server.
