This application is currently using definitions from the MIL-STD-2073-1E, change 3.
The requirements cited in these tables will be defined by use of the codes associated therewith. When using these codes, a symbol must be used in each digit position in every field of the total code. To distinguish between alphabetical and numerical "0" and "00", the numeric characters shall be designated as blank fields and the alphabetic characters as "O" and "OO". When none of the requirements of the table apply, one of the following codes shall be used:
a. Use of blank numerical codes to indicate that the field does not apply to the package described by the code.
b. Use of codes "X" or "XX" (depending on the number of digit spaces in the code field) indicates that the requirement is contained within the method of preservation.
c. Use of codes "Z" or "ZZ" (depending on the number of digit spaces in the code field) to indicate that supplementary or special requirements apply which are not represented by the code symbols. When either of these symbols are used in an acquisition document, details of the requirement shall be provided with the document.