Processing Code
What it is
Three two-digit subfields: transaction type, account type debited ("from"), account type credited ("to").
The processing code says what the transaction is. Positions 1–2 are the transaction type (00 purchase, 01 cash withdrawal, 09 purchase with cashback, 20 refund/credit, 30 available-funds inquiry, 31 balance inquiry, 40 cardholder accounts transfer). Networks refine these tables. Positions 3–4 and 5–6 give the "from" and "to" account types (00 default, 10 savings, 20 checking, 30 credit).
Issuer hosts branch on this field early: a 20xxxx refund follows a completely different posting path than a 00xxxx purchase.
Example values
000000Goods/services purchase, default accounts.013000ATM cash withdrawal from credit account.200000Refund (merchandise return credit).Specification reference
Defined in ISO 8583:1987 (base). Card networks publish their own dialect tables (Visa BASE I / V.I.P., Mastercard CIS) that refine formats, lengths, and — for private-use elements — the entire layout. Always confirm against the interface specification you implement.