The difference between SUPPORTS and MANDATORY is that MANDATORY will throw an exception if it is invoked without a Tx context. Take a look at the following fragment from Mikalai Zaikin Notes for SCBCD 5.0:
The container must invoke an enterprise bean method whose transaction attribute is set to MANDATORY in a client's transaction context. The client is REQUIRED to call with a transaction context.
If the client calls with a transaction context, the container performs the same steps as described in the REQUIRED case.
If the client calls without a transaction context, the container throws the javax.ejb.EJBTransactionRequiredException. If the EJB 2.1 client view is used, the container throws the javax.transaction.TransactionRequiredException exception if the client is a remote client, or the javax.ejb.TransactionRequiredLocalException if the client is a local client.
(The only two Tx types that will throw exception are NEVER and MANDATORY.)
So, you are right, according to MZ notes:
"If the client calls with a transaction context, the behavior is similar to REQUERIED" --> "SUPPORTS" AND MANDATORY.