Retrieval Reference Number (RRN)
What it is
Reference assigned by the acquirer to identify the transaction across its whole lifecycle — auth to clearing to dispute.
The RRN is the human-visible transaction reference: it is printed on receipts, quoted in disputes, and used to retrieve the original transaction in chargeback processing. A common (not mandated) composition is YDDDHHNNNNNN — Julian date and time plus the STAN.
Example values
621414320512Composed from Julian date 6214, hour 14, and STAN 320512.DE 37 on the wire
Bitmap position
DE 37 is bit 37 of the primary bitmap: byte 5, mask 0x08. A primary bitmap with only DE 37 present reads 0000000008000000 in hex.
Length
DE 37 is fixed length: always 12 characters, with no length prefix. A parser reads exactly that many and moves on, so shorter values are left-aligned and padded with trailing spaces.
Size in the message
At most 12 bytes when the message travels as text (ASCII or EBCDIC).
Format an
Format an means alphanumeric: letters and digits.
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.