What is the purpose of a UML Qualifier?

Question

I can't understand what a qualifier is in UML. For example, in the scanned page, there's a qualifier named "SharedRegister" that has "state" and "plateNumber" in it.

Why not just used a class called SharerRegister and put "state" and "plateNumber" in it? What does it offer over a class exactly?

(Image from Schaum's Outlines, UML 2nd edition. Simon Benett. Pg 108)

Answer 1

state and PlateNumber act like a key with which the related class can be accessed. If you didn't use a qualifier you would have a relation between the classes with multiplicity of * but would not know exactly how they are related. Here, you know that class ShareRegister, by knowing a specific value for state and PlateNumber can access 0 or 1 instances of CarSharer.