The payer ID is the number used for electronic filing for claims. Most insurance companies list this information on the back of the card near the filing address (though it can be anywhere). It's also called a payer number or an EDI number and is usually 5 digits.
In AC, if you know this number, you can search by it and it will find all the carriers under that number (for those plans that have different names) and it kind of validates the migration of that particular insurance. You can, however, add an insurance without this.
Hope that helps.
Amber
