What it is
The Application Identifier (AID) uniquely names one payment application on the chip. It is built from a 5-byte Registered Application Provider Identifier (RID) assigned by ISO to the payment scheme, optionally followed by a Proprietary Application Identifier Extension (PIX) that distinguishes products within that scheme.
During card selection the terminal reads the list of AIDs the card offers (from the PPSE / PSE directory) and matches them against its own list of supported AIDs to decide which application to run.
Where it appears
Read from the card during application selection (SELECT / PPSE), before GET PROCESSING OPTIONS.
Specification reference
Defined in: EMV 4.x Book 3 (Application Specification), Annex A — Data Element Dictionary
Example values
A0000000031010Visa credit/debit (RID A000000003 + PIX 1010)
A0000000041010Mastercard credit/debit
A000000333010101UnionPay debit