For all extensions the following numbering scheme is valid.
a matches to the minimal Joomla-Version 2.x.y. can be used with Joomla! 2.i.j we assume that the user is using the latest version of Joomla!
b is an internal number, with changing this number we implement bigger changes and it is possible that we may also change database tables.
c is for bugfixes, we don't change database tables with such an update.
It is possible that a 2.x.y will also work under a Joomla! 3.n.m, but we haven't tested this combination. The first tested version of a babioon extension for Joomla! 3 would be named as 3.0.0.