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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.