2.1

Support for linking customers to transactions using the new CUSTOMER_ID request parameter.
A new CARD_ON_FILE tender type to allow charging a previously-stored card instrument. Learn more about how to use Card on File with Square: https://squareup.com/help/us/en/article/5770-use-card-on-file-with-square-register.
Removal of oauth check. As of Square Register 4.53 on iOS, you no longer need to guide merchants through the OAuth flow to accept payments with Register API.