Virtual rate depends on the actual rate at a certain ratio (percentage, fixed). Changing the actual rate prices entails a change in the virtual one.
Virtual rates are used to create special offers and currency conversion.
As a virtual rate you may make such popular deals as: non-refundable rate, Early booking, 3 nights for the price of 2 (33.3% difference), the same day booking (LastMinute) any other promotions. Note that the virtual rates are designed for price increases (+%) too, for example, for holidays offers.