The Input Mask defines the format customers must type for a zip/postal code in that state. It uses three placeholder characters: 9 = a digit, a = a letter, * = letter or digit. For example 99999 for US zips or a9a 9a9 for Canadian postal codes.

When a customer selects that state in an address form (checkout shipping address, the delivery-area search) the zip-code box is masked accordingly, which prevents badly formatted entries. Leave it blank to accept any format.