3DS Method
The hidden first step of 3-D Secure 2, in which the card issuer's server gets to inspect the shopper's browser before authentication proper begins.
Once the merchant's 3DS Server knows which card is involved, it looks up the 3DS Method URL that the issuer's Access Control Server has registered. That page is loaded in an invisible iframe in the customer's browser, where scripts collect screen size, language, time zone and whatever else adds up to a device fingerprint. The iframe gets ten seconds. The authentication request then goes out with a flag, threeDSCompInd, saying whether the step completed (Y), failed (N) or wasn't available (U).
The spec calls it optional; in practice it decides a lot. The fingerprint is what lets an issuer recognise a returning customer and wave the purchase through without a challenge. Merchants who skip it to make checkout feel snappier tend to get more one-time codes and fewer completed sales, which is rather the opposite of the plan.