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A coat face is a face that replaces (acoustically) the covered part of the face it is assigned to.

That means:

1. A onefold coat face always looks in the opposite direction as the containing face. For example, onefold coat faces are used as parts of columns which are not considered to be in the room - like the bottom face of a column which stands on the floor. In this case neither the covered part of the containing face nor the surface of the coat face is counted when the program calculates the room surface.
2. A twofold coat face always looks in the same direction like the containing face. Twofold coat faces may be used as windows, doors, pictures on the wall etc. The surface of such a face replaces the part covered of the containing face. If a face has a high absorption(wall) and you put a coat face on it with a low absorption(window) then the covered part of the wall face does not count and it is replaced by the acoustic properties of the window face, so the average absorption will be reduced. Worth to be mentioned is the fact that coat faces must be aligned (meaning inside and in the same plane) to the containing face. Create such faces using "RMB on picked face/Vertex On Face" or align them using "RMB on picked face/Align Coat Face". Otherwise you will get an error message when checking the data.