Why are there restrictions using JNDI as the connection type when using JDBC activities in the transaction group?

Environment:
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TIBCO ActivMatrix BusinessWorks 5.7.x, 5.8.x,5.9.x,5.10.x,5.11.x.
All Operating Systems

Symptoms:
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In TIBCO ActiveMatrix BusinessWorks Process Design Guide -- > Chapter 11 Transactions -- > Configuring JTA User Transaction Transactions.

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JDBC Connections used by JDBC activities in the transaction group must be configured to use JNDI as the connection type and specify the data source in the application server.
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Reason:
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With JTA UserTranscations, transaction manager takes care of enlisting  and delisting resources. For this,  transaction resources (like JDBC connections) need to be available through JNDI lookup. If you do not have JNDI lookup, the application needs to do most of transaction manager's work.