In the <persistence-unit> tag, you specify the name attribute and transaction-type attribute.
You specify the name of the persistence unit to be defined. The name specified here is referenced from the unitName attribute of @PersistenceUnit or @PersistenceContext in the case of annotations. Also, in the case of the DD, the name specified here is referenced from the <persistence-unit-name> tag under the <persistence-context-ref> tag or under the <persistence-unit-ref> tag.
You cannot omit the name attribute. Also, when the JPA is used with Application Server, you cannot specify null in the name attribute. Specify a string of at least 1 character.
In the persistence unit to be defined, you specify whether the JTA will control the transaction or whether the application will control the transaction by using javax.persistence.EntityTransaction.
Note that if the transaction-type attribute is omitted, the default value is JTA.