Are they referring to the nozzle orifice or are they talking about the diameter of the nozzle? Does it mean the inner diameter of the valve like in a globe valve?
Can't improve on what you have except to say: If you don't ask for a full port valve; you will get an internal diameter smaller than the ID of the piping.
A 12 inch valve means it is designed to fit with nominal 12 inch pipe. Most 12 valves are flanged so these valves would also be flanged to match as 12 piping flange. They would also be sized to match the appropriate flange rating as in 125#, 250#, 150#, 300# 600# and so on. As to the opening of the valve that will vary depending on the type of valve. For a gate valve it will be very close to 12 inch in diameter. For a globe valve it may be a bit smaller. In some cases you will see what it called a reduced port valve where in internal opening is smaller. This is more common with large ball valve where the smaller ball and opening makes the opening/closing torque less. These valves also tend to have a venturi shape on the inlet and out leading to be ball so the pressure drop is closer to a full opening valve.
