Hi, not sure specifically about X5s, but assuming they are like all the other BMWs I've owned, the air filter is used to filter engine air. The microfilter, also known as the cabin microfilter, sits in the engine compartment near the firewall upper, and filters the air coming into the passenger cabin.
Does that help?
“Character is doing the right thing when nobody’s looking. There are too many people who think that the only thing that’s right is to get by, and the only thing that’s wrong is to get caught.”
- J.C. Watts Jr.