Generally there are three versions of magnetic contacts available. Normally open (type A), normally closed (type B) and change over contacts (type C).
You can find a short description at each product sub page (e.g. 100 10 34 A).

Normally open (type A) means:

Window is closed -> Magnetic contact is closed
Window is open  -> Magnetic contact is open

Normally closed (type B) works inverted:

Window is closed -> Magnetic contact is open
Window is open -> Magnetic contact closed