What makes someone able to file for tax exemption?

I think they have to be part of a 501c non-profit company. A lot of the retail store sometimes have list of those non-profit corporations or companies on file. When they make purchases if they are on that list, the tax is removed from their purchase. Some people make purchases for supplies for their schools, churches or other non-profit organizations. Those organizations are exempt and dont have to pay taxes. But of course they have to be registered and have the 501c letter of exemption. I used to worked in retail years ago, and we had lots organizations that would come in to make purchases and were exempt from paying taxes. Its not uncommon at all.