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
Model by Face panel
 (Floor by Face).
 (Floor by Face).  Multiple Selection panel
Multiple Selection panel
 (Select Multiple) to disable it. (It is enabled by default.)
 (Select Multiple) to disable it. (It is enabled by default.) If the Select Multiple option is cleared, a floor is placed on the mass floor immediately.
The cursor indicates whether you are adding (+) or removing (–) a mass floor.
 Multiple Selection panel
Multiple Selection panel
 (Clear Selection).
 (Clear Selection).  Multiple Selection panel
Multiple Selection panel Create Floor.
Create Floor.