Transaction attribute Supports is a kind of good for nothing attribute. If the method is called with an existing transaction context then it will run within that else with an unspecified context.
In the case of MDB there is no client who can propagate the transaction context and hence is not really applicable. Container invoke the onMessage of an MDB. This is my understanding.
-Sethu [ October 25, 2005: Message edited by: Sethu ]