Definition of class 100

Hello friends,
I have a question about class 100.
We have a count particals at my company which count particalsat size of 0.3/0.5/5
Its counts the 3 particals but we look only 0.5 particals?
I don’t understand why class 100 is talking only about 0.5 micron and not 0.3/5 micron?

I will be very thankfull if you could answer me.

Thank you