JBoss.org Community Documentation
In JTA, the identity of transactions is encapsulated in objects implementing the javax.transaction.xa.Xid interface. The transaction ID is an aggregate of three parts:
The format identifier indicates the transaction family and tells how the other two parts should be interpreted.
The global transaction id identified the global transaction within the transaction family.
The branch qualifier denotes a particular branch of the global transaction.
Transaction branches are used to identify different parts of the same global transaction. Whenever the transaction manager involves a new recoverable resource in a transaction it creates a new transaction branch.