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Using guestIdentifier values

The Toast platform uses the guestIdentifier value for fraud evaluation. The guestIdentifier value is a unique identifier for a restaurant guest associated with a credit card payment.

You use the same identifier for a guest even when that guest uses different methods of payment. The guestIdentifier value should be unique per guest but not unique per credit card. In other words, if one guest uses two credit cards, you use the same guestIdentifier when authorizing both cards. Examples of values for the guestIdentifier field include a guest's email address or a randomly-generated GUID that is unique per guest.

If you use the same guestIdentifier value repeatedly on many card authorizations for multiple guests, authorizations using this guestIdentifier are likely to be declined because of the suspicious activity. A guestIdentifier value of 0 will cause credit card authorizations to fail.