National Insurance number is a UK number which is given to everyone when they are 16 - it is a unique government tax identifier which stays with that person for life.
As each number is truly unique, we use this field to prevent the same employee being entered twice on the system.
National Insurance number is also known as Social Security number and whilst other countries may call it something else, every employee has a unique TAX ID hence this field is used for this.
*It is not possible to remove the dependency on this field - if you truly do not want to enter a National Insurance or Social Security number then simply enter a unique value in this field for each employee.*