I don't think there is a difference, they used to make the small business pins RED so you can see them more easily. I didn't even notice the P pins until you pointed it out. Not many by me but they look like the typical small business shops.
You have to buy two identical items, purchase one with debit, one with cash, need a receipt for each to show if they are charging any fees or giving any discounts. Usually also requires a picture of any signage that says they are charging fees.
They are pretty easy but it is location dependent. Some small stores don't like to give receipts. Print out the guidelines because there are many exceptions. Try one, get the hang of it, see how it goes.