PAN, Extended
What it is
Legacy field for PANs that do not fit DE 2 conventions. Effectively unused in modern networks.
DE 34 on the wire
Bitmap position
DE 34 is bit 34 of the primary bitmap: byte 5, mask 0x40. A primary bitmap with only DE 34 present reads 0000000040000000 in hex.
Length
DE 34 is variable length (LLVAR): a 2-digit prefix gives the length of the value in characters, from 0 to 28. A 12-character value is sent as 12 followed by the value. In ASCII the prefix is the bytes 31 32; dialects that pack lengths as BCD send 12 instead.
Size in the message
At most 30 bytes when the message travels as text (ASCII or EBCDIC), or 29 bytes in binary dialects that pack the length prefix as BCD.
Format ns
Format ns means numeric and special: digits plus special characters.
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.