JPA 2.0 specification section 7.6.3.1 Requirements for Persistence Context Propagation:

"
Persistence contexts are propagated by the container across component invocations as follows.
If a component is called and there is no JTA transaction or the JTA transaction is not propagated, the
persistence context is not propagated.
"

For the case of a SFSB (with an extended persistence context XPC) method that is not in a JTA transaction, invoking a SLSB (with a transactional entity manager PC) method that requires a transaction. The SLSB method should not use the extended persistence context but should instead start a new persistence context instead for the duration of the SLSB transaction (will end at the method end).