It usually requires at least a 20% down payment and short terms (1 to 2 years), designed as a bridge: the goal is to return to an A lender once the file allows it. A regulated B lender is not a private lender, whose cost is higher still.
Before accepting a B loan, a broker checks whether an A lender would take the file with a different structure (co-signer, larger down payment, income documented another way). A B loan is a tool, not a destination.