what is section on 28/12.5/15 on 12 inch rim?

No Not a trick question. I really need to know? The section isnt always what one might think it would be or reported to be by a manufacture. I know from experience.Most folks seem to be using a 10 inch rim and thats what rim is used when given specs.on a mfgs or retailer web site. I have a 12 inch rim under my Dart