(DataBasics.DataSources[""].Transactions).

Normally, When BusinessLayer.TransactionBegin is called a DbTransaction Object is added to the collection with a default key value. This default transaction will be used.

When do I need simultaneous transactions ? I mean, When will be more than one transactions in that collection. So I will follow a key.

The beauty of StrataFrame is that you can have different business objects saving on different transactions.  When you call the Save you can supply the transaction key.  So if you start more than one transaction you will have more than one in this collection.  This allows some BOs to be saved on Transaction A, others on Transaction B, and some not on a transaction if you have this need.