A slice can be thought of as a representation of a theoretical floor level. Slice floorplates provide a way to translate the 3D geometry of a mass group into something that can be used to generate the individual floors of a building. Slices can be converted to space boundaries or polylines, and then to walls.
Like mass groups, slices are chosen and manipulated by an associated marker, the slice marker. When you attach mass groups to the slice marker, the slice marker creates the perimeter geometry of the building, known as a floorplate. Anytime you change the mass groups attached to the slice marker, the floorplate is updated.
If you attach mass elements instead of mass groups to the slice, the operational aspects (additive, subtractive, and intersection) of the mass elements are not recognized by the slice. To have these aspects recorded by the slice, attach the mass group.
After you create floorplates, you can convert slices to space boundaries or polylines, and then generate walls. You cannot create walls directly from a slice. The slice is simply a way to mark each floor level of the mass group.