The opt-out date is the date when you opt-out of paying VAT in the origin country. You opt-out from the origin country to the destination country. Keep in mind this term is not an official term used by tax offices. For example: In the image below you have the Czech Republic, if you fill out the opt-out fields below for every country it means that from that date onwards, you won't pay VAT in the Czech Republic anymore and instead you will pay in those countries. Read more about opt-out.
The opt-out date is the date you decided to pay VAT in the destination country. This field is for the destination country.