IEC 61400-1 Vhub vs. Vref

[tlewin]

In section IEC 61400-1, on p. 45 Table 1 specifies Vref (10 min. avg, at hub height) for wind turbines, based on class. Later throughout chapter 6, the speed Vhub is used. How are these two speeds related? I cannot seem to find a definition of Vhub (only that it is velocity at hub height).