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).DE 3 on the wire
Bitmap position
DE 3 is bit 3 of the primary bitmap: byte 1, mask 0x20. A primary bitmap with only DE 3 present reads 2000000000000000 in hex.
Length
DE 3 is fixed length: always 6 digits, with no length prefix. A parser reads exactly that many and moves on, so shorter values are right-aligned and padded with leading zeros.
Size in the message
At most 6 bytes when the message travels as text (ASCII or EBCDIC), or 3 bytes in binary dialects that pack digits as BCD.
Format n
Format n means numeric: the digits 0–9 only.
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.