ChangeFlagElementID helps Process Tasks know when an input value has been changed by the user. For example, a Process may only UPDATE rows when the user actually changed something, and otherwise skip the UPDATE.
Set ChangeFlagElementID to the ID of an InputHidden element. Then, if the user changes this input element, the hidden element's value will become "True". You can test for this value in a Procedure.If.
When using ChangeFlagElementID with an Input element that's under a DataTable, be sure to put the InputHidden element under a DataTableColumn. Process input from individual rows under a Procedure.RunDataTableRows.