So there are 3 kinds of "debit cards" in Canada:

1. Debit Mastercard/VISA. These are Debit Cards that use the Mastercard/VISA communication system to process transactions. While they are not "Credit" cards because you are using cash in an account that is your money, they rely upon the VISA/Mastercard system and merchants will be charged the Mastercard/VISA fee like a Credit Card.

2. Interac Debit Card. Interac was the first company to offer a debit card type system in Canada, and they are the traditional bank card. These cards use the Interac system (so does eTransfer) and Merchants are charged by Interac for using the system. Its typically less than Mastercard/VISA, which is why you see these "Debit Card only" signs.

3. Mastercard/VISA and Interac hybrid cards. These are newer and combine both Mastercard/VISA and Interac cards in one. The merchant can choose how they want to proceed.

Most of these "Debit" only signs are really saying "Interac only", but because for 30 years Interac was the only provider of Debit cards in Canada, it became the common vernacular to say "Debit" when you mean "Interac".