Acquiring Institution Country Code
What it is
ISO 3166 numeric country code of the acquirer — basis for domestic vs cross-border qualification.
DE 19 on the wire
Bitmap position
DE 19 is bit 19 of the primary bitmap: byte 3, mask 0x20. A primary bitmap with only DE 19 present reads 0000200000000000 in hex.
Length
DE 19 is fixed length: always 3 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 3 bytes when the message travels as text (ASCII or EBCDIC), or 2 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.