With JPA you can speciy two types of transactions :
1- RESSOUCE_LOCAL : available in and outside a JEE environment, and rely on the underline Data Source you use. For example you can use this type to work with database transaction.
2- JTA : for Java Transaction API it's only available in a JEE environment. This kind of transaction is more rich than the first one. You can have a transaction with different Data Source for different types. For example between a database and LDAP.
When you are in a JEE environment, the JTA is the default value. However, RESSOUCE_LOCAL is the defaut value in a SE environment.