To create floors from a mass instance, use the Floor by Face tool or the Floor tool. To use the Floor by Face tool, you first create mass floors. Mass floors calculate floor areas in a mass instance.
To create a floor from a mass floor
Model by Face panel
Multiple Selection panel
If the Select Multiple option is cleared, a floor is placed on the mass floor immediately.
How to
The cursor indicates whether you are adding (+) or removing (–) a mass floor.
Multiple Selection panel
Multiple Selection panel
Create Floor. 
Cursor highlighting mass floor

Created floors