Track bike, road fixed, cross fixed, two single speed mountain.
Just to clarify, what dolface said.
Comes from mountain bike sizing, where radically sloping top tubes and raised seat masts made conventional sizing standards questionable to follow. The virtual top tube measurment is the length of the imaginary line between the center of the headtube and the center of the seattube, level with the ground, and about 2-3cm down from the top of the headtube, where a flat top tube would attach. Other virtual measuremtns follow, like those for elevated chainstay bikes etc...