Track 3 Data
What it is
Magnetic-stripe Track 3 (ISO 4909) — historically used by some debit networks; effectively obsolete.
DE 36 on the wire
Bitmap position
DE 36 is bit 36 of the primary bitmap: byte 5, mask 0x10. A primary bitmap with only DE 36 present reads 0000000010000000 in hex.
Length
DE 36 is variable length (LLLVAR): a 3-digit prefix gives the length of the value in digits, from 0 to 104. A 12-digit value is sent as 012 followed by the value. In ASCII the prefix is the bytes 30 31 32; dialects that pack lengths as BCD send 00 12 instead.
Size in the message
At most 107 bytes when the message travels as text (ASCII or EBCDIC), or 54 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.