See MARC21 for descriptions of all character positions 00-23. See CODES for UNBIS coded values.

Fixed field elements entered in the Leader include the following:

Positions 00-04: Record length. Computer-generated, five-character numeric string that specifies the length of the entire record.
Position 05: Record status.
Position 06: One-letter code indicating the type of item being described. If the item is essentially the equivalent of a print item but in electronic form, use the same type code you would use for the print version. For UN documents, use the code «a» (language material) regardless of whether the format is printed material or an electronic resource.
Position 07: One-letter code indicating the bibliographic level.
Position 17: One-character code indicating the encoding level, i.e. the status of completion or fullness of the bibliographic record.
Position 18: One-letter code indicating the cataloguing form of the record.

Examples:

000 02227cam 2200517#a 4500
where:

Positions 00-04: 02227 is record length;
Position 05: c is record status (corrected or revised)
Position 06: a is type of the item (language material)
Position 07: m is bib. level (monograph/item)
Position 17: # is encoding level (full level)
Position 18: a is cataloguing form (AACR 2)

000 01063cem 2200325 a 4500
where:

Positions 00-04: 01063 is record length;
Position 05: c is record status (corrected or revised)
Position 06: e is type of the item (cartographic material)
Position 07: m is bib. level (monograph/item)
Position 17:   is encoding level (blank)
Position 18: a is cataloguing form (AACR 2)