Thanks for bringing it to our attention. This seems to be an error in the docs - it is used by other payment gateways as well. Specifically, I can see it used by these gateways as of Matrix 5.5.4.1:
Braintree
DPS (Payment Express)
PayPal Express
PayPal Payflow Pro
SecurePay Direct Post
PayWay
Which is most of them. Notably, it does NOT seem to be sent by the SagePay gateway for some reason.
I’ve fixed that in the docs now to just refer to Payment Gateways in general. Thank you.