10.3.3. About the Presumed-Abort Optimization
If a transaction is going to roll back, it can record this information locally and notify all enlisted participants. This notification is only a courtesy, and has no effect on the transaction outcome. After all participants have been contacted, the information about the transaction can be removed.
If a subsequent request for the status of the transaction occurs there will be no information available. In this case, the requester assumes that the transaction has aborted and rolled back. This presumed-abort optimization means that no information about participants needs to be made persistent until the transaction has decided to commit, since any failure prior to this point will be assumed to be an abort of the transaction.