This page shows the track of the popular Sella Ronda Bike Day.
It doesn’t really matter where you start and/but course direction can change from year to year, i.e clock- or anti-clockwise.
However, I believe there is no (official) rule stating that you cannot go against the “advised” direction for the official event, but it would be silly to do so.
This loop is also part of the even more popular, but harder to enter, Maratona dles Dolomites. That starts just north of Corvara, in La Villa and includes two ascends of the Campolongo.
Because of those, the course direction of the Sella Ronda part for the Maratona is always clockwise. The counter-clockwise version of the Sella Ronda Bike Day, has the Campolongo as a descend.
Where it is free to enter and does not require you to register for the Sella Ronda Bike Day, the Maratona is neither.