From the brief description (on the Burson page) it looks the same thing. As with any other amp with a selectable gain switch, like the O2 and presumably countless others. The description "The new VOS provides 3 level of gain to the preamp function from 7db to 18db" doesn't indicate there's anything more to it than that.
Alright, from my understanding (im not expert on electronics, know some basics only)
It depends on the design, for example. A)It can be a true gain switch, and it adjusts the amplification level of the circuit
B)Is just a potential divider to cut the signal level.
C)An output impedance selector, which by default will deliver differing signal amplitudes according to setting.
