here is something I found in the JPA faq
Q. Can I use a resource-local EntityManager in my web application?
Yes, you can use both resource-local as well as JTA EntityManager in your web application. The same applies to an EJB application as well. But, we recommend you use a JTA EntityManager in your web and EJB applications and [b]use a resource-local EntityManager in your Java SE or Java EE client application. [/b]
There is no reason given though.
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