Seatbelt sensor tells the system someone is there, the mat and seat sensor are an attempt to make sure the passenger is the right size to use the airbag. SO called smart airbags.

Meh, I'd take them out if it wasn't such a freeking liability. Hate airbags. They are there to total cars not save lives. If your belt is properly on you dont even hit them. (In my experience....)

Dave

That came later. The E36 still used "dumb", fixed charged airbags that explode the same way for are a child or an adult.

The American-market seat belt buckle switches determine the threshold and moment to fire the airbags. The issue is that the U.S. NHTSA requires the airbags to protect the occupants whether they are wearing seat belts or not. The airbag has to go off in milder crashes and go off earlier, if someone isn't restrained and slowed by a seat belt. Conversely, if they are, the airbag need not go off until a more severe crash and can fire later. The American seat belt buckle switch adjusts the firing, accordingly, to protect both belted and unbelted occupants.

European E36s don't have the troublesome SRS buckle switches since people are expected to wear their seat belts.

The passenger side occupancy mat only determines if someone is in the seat: to avoid firing the airbag when the seat is empty.