You apply slab edges by selecting horizontal edges of structural floors. You can also select model lines.
You can also access this tool by clicking Structure tabFoundation panelSlab drop-down (Floor: Slab Edge).
You can also click model lines. As you click edges, Revit treats this as one continuous slab edge. If the slab edge segments are created in the same placement session and meet at corners, they miter.
Slab edge added to a structural floor
Tips
After placing slab edges, you can modify them in several ways.