Quasi-cash
Card purchases of things that are effectively money, such as casino chips, foreign currency and traveller's cheques, which card networks therefore treat almost like cash withdrawals.
Network rules draw a line between buying goods and buying money in another form. Casino chips, money orders, foreign banknotes and loading certain prepaid cards fall on the far side of it. Visa and Mastercard group them under merchant category code 6051, and the transactions must be flagged so the issuer can see what is going on.
The issuer then decides how to treat the purchase, and most choose to treat it as a cash advance, with a fee and interest from day one. When bitcoin spiked in early 2018, several large US banks stopped crypto purchases on credit cards altogether, citing exactly this risk. The boundary is not always obvious: the same crypto exchange can show up with different codes depending on which acquirer it uses.