I think what is not clear is what it means when previously is used.
For example, SessionBean1 (which creates a transaction) is calling another SessionBean2 method(which also uses transaction). Here previously means [for SessionBean2] is SessionBean1's transaction. If REQUIRED is used in SessionBean2, SessionBean2 joins to the transaction of SessionBean1's transaction. (Any rollback from SessionBean2 will rollback SessionBean1's transaction because they are same.)
In our case since MDB cannot be called from another bean there is no previous transaction, so most of the attributes become invalid.
