Dynamic currency conversion (DCC)
The "would you like to pay in your home currency?" prompt on a terminal abroad, to which the right answer is almost always no.
The terminal reads the card's BIN, notices it was issued in another country and offers to convert the amount on the spot. It sounds like a service. The rate, however, is set by the acquirer or the terminal provider, and the markup often runs to several percent – well above what the card network itself would charge. The merchant gets a cut, which explains why staff sometimes push the question themselves.
The EU lost patience: since April 2020, Regulation 2019/518 requires the markup to be shown as a percentage over the ECB reference rate before the customer chooses. ATMs in tourist spots still do their best to word things so that "accept conversion" looks like the safe button. Pick the local currency and the card network does the conversion at a noticeably better rate.