100% biodiesel is called B100. It is also known as "neat" biodiesel.
20% biodiesel blended with 80% petroleum diesel is B20. 5% is B5, 50% is B50, etc.
Can I go Back and Forth Between Regular Diesel and Bio-Diesel?
YES! Biodiesel can be splash blended in your fuel tank in any proportion with petro-diesel. Some fleets blend their own B20 or buy from distributors pre-blended to keep costs down. Most commuters I am aware of are using B100 but occasionally use a bit of petro-diesel only when they are low on fuel and far from a biodiesel pump.