Bitmaps
Within ISO 8583, a bitmap is a field or subfield within a message which indicates which other data elements or data element subfields may be present elsewhere in a message.
A message will contain at least one bitmap, called the Primary Bitmap which indicates which of Data Elements 1 to 64 are present. A secondary bitmap may also be present, generally as data element one and indicates which of data elements 65 to 128 are present. Similarly, a tertiary, or third, bitmap can be used to indicate the presence or absence of fields 129 to 192, although these data elements are rarely used.
The bitmap may be transmitted as 8 bytes of binary data, or as 16 hexadecimal characters 0-9, A-F in the ASCII or EBCDIC character sets.
A field is present only when the specific bit in the bitmap is true. For example, byte '82x is binary '1000 0010' which means fields 1 and 7 are present in the message and fields 2, 3, 4, 5, 6, and 8 are not present.
Read more about this topic: ISO 8583