There is a topic going on about the case on the definition of IfcBuildingStorey which you can read here.
As a result of those discussions and the few (but growing) POLL results, I would like to suggest this enhancement for the IFC4 Schema.
As @jonm suggested, the schema needs a type enumeration for the FFL and SSL. This option would offer more flexibility in choosing where the storey is defined by which use case and still being able to exchange correct data. As the poll suggests, even though the definition in the current schema is at the SSL, I believe users would like to have the possibility to define this parameter also at a different level.