Two main factors determine what Credit Grade you are. The interest rate a lender will charge depends on these two factors. In plain English, A Paper refers to borrowers with excellent credit which means no late charges whatsoever for the last 7 years. Any Bankruptcy dismissal or discharge should be over 3 years ago with re established credit. These are general guidelines and may vary from lender to lender.
Exceptions are always possible with strong compensating factors.
D' papers refers to what is known as hard money loans which are mostly based on the equity in your home and not on your credit. The higher the LTV, the more stringent the lenders become on credit and income. 28 is called the front ratio and 36 the back ratio. They are also known as top and bottom ratios. However, lately these ratios have been stretched.