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Here is how it works. If you bind the outer test to a data source with 3 rows and at the same time you bind the nested test to a data source with 2 rows it will execute as follows:
On the first iteration of the outer test the middle test will be executed 2 times. On the second iteration of outer test the middle test will be executed again 2 times, the same will happen on the 3rd iteration.
With that in mind, please give us more detailed information what exactly do you want to achieve.
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