[boost] Boost version meaning

Hi, I would like to know what is the meaning of the 3 numbers on the Boost version X.Y.Z. When X changes, i.e. when we could have a Boost version 2.0.0 Which number means a complete rewriten? Which number means an incompatible evolution? Which number means new functions? Which number means major bugs corrections? Which number means only minor bugs corrections? Is this written somewhere? If we were to version individual Boost libraries which will be the apropiated schema? Should all the libraries follow the same schema? What do you think of? X: complete rewriten or incompatible evolution Y: new functions or major bugs corrections Z: minor bugs corrections Thanks, Vicente