Mortgages are typically classified based on the payment risk that the lender faces.

Prime mortgages (also known as “A” deals) entail lower chances of the borrower defaulting than non-prime mortgages (aka., subprime or “B” deals). 

“B” mortgages are riskier so lenders rely heavily on the equity in the subject property and/or on charging rate premiums to mitigate that risk.