Significance of the 'AS 2.0 class' in linkage

I’m doing a tutorial on classes and the tutorial says it’s VERY important to put the name of the class in the AS 2.0 class: part of the linkage for a movie clip, but doesn’t explain WHY it’s VERY important. I’ve noticed that whether it’s filled out or not, the results are the same. I can still create a new instance of the class and still use its methods.

here’s a link to the tut:
http://www.wildform.com/support/tutorials/AS2Class/

if anyone has a better resource for learnign classes, please let me know.