ID 90 = staff passenger with 90% discount on fare.
ID 50 = staff passenger with 50% discount on fare.
Other ID's. Yes ID 75 = 75% discount on fare.
<<What is a non-revenue passenger? Can a normal person (as in not working in the aviation industry) be a non rev passenger? >>
A non-revenue pax is just what it says - somebody from whom the airline recieves no payment for travel. A example of such pax would be travel agents whom the airline wish to 'sweeten up'.
There are other examples, but generally 'normal' people have to pay their way.
Hope this is of help