Reservation for places
As stated in the forum by Dirk Bräuer (1) the element may be extended by a new optional attribute reservation.
It could be extensible with a pre-defined enumeration list, e.g. notPossible, possible, recommended, compulsory.
It is needed to define a mandatory reservation for "places" in general and for bicycles in special.
It should be only used in the context of trainParts, not in vehicle.