Yes: Simply overwrites and wipes any data that might be on the destination field to populate. So even if the local field has nothing, the destination field gets wiped blank.
No: If there is data already in the destination field then it does nothing.
Ignore if blank: If the local field to fetch has no data, then nothing will happen to the destination field.
Append to Data: It will add the value of the local field to fetch onto the end of whatever is currently in the destination field. This would mean if you kept triggering this rule the data in the destination field would get longer and longer.