It's both. I got 3ds triggered on a vendor I used all the time, but one time I bought from it while I was in Vietnam - hello secondary verification screen. Also

It's both. I got 3ds triggered on a vendor I used all the time, but one time I bought from it while I was in Vietnam - hello secondary verification screen. Also happened for vendor that I had no issues with, but when I used my card from not-US bank - secondary verification screen. Suspicion on the vendor usually, at least with my banks, triggers a fraud alert that I have to respond. 3DS on the other hand is to verify that the person making a payment is the rightful cardholder. Masterpass also doesn't always redirect to that verification screen.