The valve on the oxygen cylinder,as used in welding and cutting equipment, is double seating to prevent leakage from the stem. This is why the oxygen valve on the cylinder should be fully opened when using the equipment.
I think the application of "double seat seal" is to prevent leakage from the shaft that opens and closes any valve.A drawing of the API 6D ball valve NPS16 CL 1500 can probably be obtained from a manufacturer. A Google search, will provide you with examples of other valves.
Respectfully, the operating shaft of a ball valve is a 'stem', and any associated seals are 'stem seals'.
The word 'seat' refers to the troublesome seal between the ball and the housing, surrounding the housing process fluid apertures, or to any of the mechanical complications in that area intended to enhance the seals' durability or resistance to fire or to abrasive materials.
So a 'double seat seal' would refer to, e.g., two o-rings of slightly different size, in two separate glands, or something like that. The term is not really definitive by itself, because 'floating' seat arrangements have a seal between the ball and the seat and another seal between the seat and the body, so they might be called 'double seat seal', or not, depending on your semantics.
The second seat on the oxygen cylinder's valve stem is commonly referred to as a 'back seat', so when you open the valve fully, it's 'backseated'.
Back seats that do more than stop stem leakage appear on refrigeration service valves, where they can be used to isolate a compressor, or to cause it to evacuate its own system.
