Just a little confuse when I learned about the aircraft classification based on approach speed. Can anyone help me to have an accuracy look about this question?
Vref is indeed defined as
Landing reference speed or threshold crossing speed.
It is normally set with reference to Vs0 which is stall speed in landing configuration. Vref must usually be greater than 1.3x Vs0.
By way of example, the POH for my PA-28 lists the stall speed with flaps down (ie. landing configuration) as 45KIAS. I tend to be around 70KIAS on final approach and 65KIAS over the threshold. 1.3x my stall speed would be roughly 59KIAS, so I have about 5KIAS safety.