Application Identifier (AID)
The hexadecimal label that names an application on a chip card, used by the terminal to decide whether a payment runs as Visa, Mastercard or a domestic scheme.
The format comes from ISO/IEC 7816-5. An AID is 5 to 16 bytes long. The first five are the RID, identifying the registered owner of the application; the rest is the PIX, which the owner assigns. A0000000031010 is Visa's credit and debit application, A0000000041010 Mastercard's. Contactless terminals start by reading the PPSE directory (2PAY.SYS.DDF01) to see which AIDs the card offers.
A co-badged card carries several AIDs, one per scheme, and their priority decides which network the terminal picks. EU Regulation 2015/751 gives the cardholder the right to choose, a detail merchants are not always eager to mention.