Hi Marius and Stefano,
as far as I know the default engine in the BaBar-like full simulation software is the "Ranecu" generator. It is also possible to use all other engines provided by CLHEP. Everything is steered via a random control mechanism which makes sure to reproduce sequences when it is needed to re-run specific jobs or even single events from one specific job. I posted already a link to a documentation on the random control package developed by BaBar (see posting: http://forum.gsi.de/index.php?t=tree&goto=7708&rid=431&S=e6e 3768b7c9b983ec4aae27f79f25413&srch=RandControl#msg_7708
)