Can someone please explain me where is it defined that (when you register the armor) 0 is helmet, 1 is chestplate, 2 are leggings and 3 are boots. In which class is it defined?

 

But then what if I want to make new kind of armor how do I define it then, or do I just do it the old way?

Can I add more numbers or how do I do it then?

If you want to add new slots you will need to figure out how to handle that yourself.

Thats where you as a dev have to design a system for handling what you wish to create

 

Remember you will have to add new containers for holding the armor and handle all that stuff as well

Ok. Thanks.