Meaning of Zone Digit

Meaning of Zone Digit: 

The numerical key to a section of a code, e.g., where a hole was punched in the X-position of a column of a punched card it yielded a value to a hole punched in the 2-position of the same column of that card. When a hole were punched in the Y-position instead of the X-position, a different value would be given to the hole in the 2-position, and the X-, Y-and (if appropriate) 0 punching positions were thus zone digits. Zone digits are used independently of other punching for control significance, etc.