Floors can contains several layers, used as a hierarchy element, e.g. put all guidelines on a new layer or all furniture objects. When you create a new floor at least one layer named "Floor plan" automatically exists.
Additionally layers have their own level, relative to the floor level.
Example: You have an upper floor with a level of 2,8 m, because the ground floor below has a height of 2,8 m. This upper floor has a layer "floor plan" with level zero. Zero in this case means, that the total height above ground is 2,8 m because the upper floor has this level. If you create a new layer in upper floor and set its level to 0,5 m, the total level is 3,3 m, 2,8m + 0,5 m. All elements you insert on this layer will automatically have a level of 3,3 m and not 2,8 m as on the floor plan.
With this mechanism you can create floor areas with different levels. If you want to create a room on such a layer please note that walls on different layers have no intersection and will therefore not create a closed room contour. In this case you have to close the contour manually with a wall type "room boundary".
The video shows a simple split level example.