Hi Paulo,
As far as I know (SQL-Traces) all add-ons share the same transaction than B1.
As suggested by Alessandro, you can also in specific cases for ce a different transaction, but be aware that it will not work in every single cases (I remid a test I did some weeks ago, for somebody who wanted to post an invoice followed immediatly by a payment: you just cannot include both operations in a user transaction, else it will break).
Regards,
Eric