It depends on how it is being sold and for what reason. BS476 Pt 22 & BS 1634 is a fire resistance and smoke control test for door, shutter and openable window assemblies and elements of building hardware to ensure that in the event of a fire the integrity of the door and the components which support the door in the frame, door locks/keepers and hinges, are maintained.
Not all fire resisting doors are required to be self closing but those which are provided to protect escape routes are considered critical for life safety and as such are required to be maintained closed at all material times and, to ensure this, fitted with suitable self closing devices.
Where a self closing device is necessary this must be of an appropriate standard to ensure that it suitable for the purpose. There are many self closing devices available for particular and specific uses but that which is necessary for a fire door should comply with BS 1154. A device is tested to this standard to ensure that essentially, it is suitable for the purpose for which it is intended, one of which is to ensure the door closes properly and fully from any open position, at a sufficient speed and force, overriding any latch.