MAP=Manifold Absolute Pressure=Referenced to vacuum
MDP=Manifold Differential Pressure=Referenced to atmospheric
No matter what you call it, it serves the same function, it is used by the ECU to calculate engine load, which the ECU uses for a few things, like which timing map to use and when to open the EGR valve as well as open/closed loop.

I read that it happened to some people that go from a 7 bolt to a 6 bolt. The MAP on the intake manifold is to check operation of the EGR system and a MAS to meter airflow