Simplify the following text for a Grade 13 native English-speaking person.
Original: Common usage has expanded the term to refer to any unique alphanumeric identifier for one of a large set of objects , however not every numerical identifier is a serial number ; identifying numbers which are not serial numbers are sometimes called nominal numbers . In data processing and allied fields in computer science , the distinction between serial and nominal numbers is an important one .
Simplified: Not every numerical identifier is a serial number ; identifying numbers which are not serial numbers are sometimes called nominal numbers .