EJB Isolation levels in ejb-j2ee-engine.xml

Can anyone explain to me why I only seem to have the option of using the 'repeatable read' isolation level in Netweaver Developer Studio?

I'm new to SAP and Netweaver but as a seasoned J2EE developer I would have expected to find read committed, read uncommitted, serializable, as well as repeatable read.

Is there some SAP or Netweaver specific reason why I don't get to choose these options, or do I need to enable them by configuring properties somewhere?