10.2.13. About Nested Transactions

Nested transactions are transactions where some participants are also transactions.

Benefits of Nested Transactions

Fault Isolation
If a subtransaction rolls back, perhaps because an object it is using fails, the enclosing transaction does not need to roll back.
If a transaction is already associated with a call when a new transaction begins, the new transaction is nested within it. Therefore, if you know that an object requires transactions, you can them within the object. If the object's methods are invoked without a client transaction, then the object's transactions are top-level. Otherwise, they are nested within the scope of the client's transactions. Likewise, a client does not need to know whether an object is transactional. It can begin its own transaction.
Nested Transactions are only supported as part of the Java Transaction Service (JTS) API, and not part of the Java Transaction API (JTA). Attempting to nest (non-distributed) JTA transactions results in an exception.