With a solid object selected in the canvas (in this example, a simple building shell), the Properties panel will show a "Use Levels" checkbox.
If the FormIt sketch already has Levels defined (see above), then checking this box will use all Levels that would intersect this shape (ignoring any that would be too high or too low).
If the FormIt sketch does not have Levels already, checking this box will create enough default Levels.(12' floor-to-floor height) to intersect the entire shape, and will automatically apply those Levels to this object.
Levels + Revit
When Levels are applied to FormIt geometry, those Levels will be sent to Revit when using the FormIt Add-In.
In Revit, you can use the FormIt levels to create Mass floors, Floor by Face, and floor plans associated with the FormIt Levels.