The Leica filters for the Leica M system are made of optically pure glass, parallel plane ground and precisely annealed, so the full performance of the Leica M lenses is retained even with a filter attached. The range is purposely limited to the filters most commonly needed in photographic practice: The UVa filter primarily performs the role of protecting the front element of the lens, and can be left permanently attached. The circular polarization filter eliminates disturbing reflections on non-metallic surfaces, and helps to increase contrast as well as provide richer color reproduction.
A characteristic of polarization filters is that they show the same effect after each 180-degree rotation. The polarization filter for the Leica M is thus supplied with an adapter with a swivel fitting, allowing it to be rotated 180 degrees to in front of the viewfinder so you can assess and control its effect before taking the shot.