You can manually assign object to a building storey in the containment palette.
You can automatically assign objects to building storeys using the 'IFC Auto Containment' tool. Running this tool will assign objects to the correct building storey based on the elevation of the object relative to the elevations of the building storeys.
If you have objects that are multiple building storeys high and you need to cut them such that each building storey has its own object, then you can use the Location Tools add-on.