AS NZS ISO 2709:2013 pdf download – Information and documentation – Format for information exchange
4.3.2 Record length (octets 0 to 4)
The record length shall be defined by the number of octets in the record, including the record label, directory, fields, and the record separator. The length is a five-digit decimal number, right-aligned with zero fill if necessary. NOTE The record length described here is a logical record length. For practical reasons relating to machine processing of data, it is sometimes necessary to divide the information into blocks.
4.3.3 Record status (octet 5)
The record status shall be defined by a single character, to be defined in an implementation International Standard, describing the status of a record, for example new or amended. In the absence of an International Standard, special agreement should be reached between the interchange partners.
4.3.4 Implementation codes (octets 6 to 9)
The implementation codes are not defined in this International Standard. Special agreement should be reached between the interchange partners.
4.3.5 Indicator length (octet 10)
The indicator length shall be defined by one decimal digit giving the number of octets of the indicators. If indicators are not used, the indicator length shall be set to zero.
4.3.6 Identifier length (octet 11)
The identifier length shall be defined by one decimal digit giving the number of octets of the identifier. The first or only character of this identifier shall always be IS1 in accordance with lSO/IEC 646 or ISO/IEC 6429. If the identifier is not used, the identifier length shall be set to zero.
4.3.7 Base address of data (octets 12 to 16)
The base address of data shall be defined by five decimal digits, right-aligned with zero fill if necessary, equal to the combined length in characters of the record label and the directory, including the field separator at the end of the directory.
The length of the tag shall be three octets. The length in octets of the other three parts in each directory entry shall be given by the directory map (octets 20 to 22 in the record label). All entries in a directory shall have the same structure.
4.4.3 Tag
The tag shall consist of three octets which specify, according to definition in an implementation International Standard, the name of any associated field.
In the absence of an International Standard, special agreement shall be reached between the interchange partners.
4.4.4 Length of field
This length shall be either
a) the total number of octets [including indicator(s) and field separator] in the field indicated by the preceding tag, or
b) zero, implying that the directory entry refers to a field whose total length is greater than the largest decimal number (n) which can be stored in the “length” of field part of a directory entry (see Note), or
c) the number of octets (including field separator) in the final part of a field which has been treated as described in b).
NOTE In the case of b), the field is regarded as being divided into a number of parts, of which all but the last are of equal length (n). Each part has a corresponding directory entry containing the tag for the field and the starting character position of the part to which the directory entry refers. A length of zero indicates that the directory entry refers to a part of the field which is not the final part and that the length of this part is to be taken as (n).
In the cases described in b) and c), all directory entries which refer to parts of the same field shall be adjacent and in sequence.
4.4.5 Starting character position
The starting character position shall consist of a decimal number giving the position of the first octet of the field identified by the preceding tag, relative to the base address of data [i.e. the starting character position of the first field following the directory is 0 (zero)].