Mass Air Flow Sensors measures both the amount of air volume and density of the air entering the engine. Mass air flow sensors include Vane type, Hot Wire, and Karman Vortex

MAP sensors, Manifold Absolute Pressure sensor, measures the absolute pressure of the intake manifold. As pressure drops, vacuum increases, which allows fuel injection duration to increase. As pressure increases, vacuum drops, which allows fuel duration to decrease.

HTH but i know my BUICK has Both a MAP and MASS (never checked the 99 RL though, sorry)