The interchange fee is paid by the merchant to the acquirer for processing transactions. The pricing for this fee can be either following the interchange ++ model or blended rates model.
The interchange ++ model provides the merchant with flexible rates, depending, amongst other factors, on the security of the transactions. This is why, it is generally preferred by merchants processing a small number of unsecured transactions. However, the exact amount the merchant is being charged is less predictable than for the blended rates pricing model.