A submit secondary datasource gives you the ability to set subject line and attachment name based on field formulas.
But I'm not sure I understand how the repeating section value is to be used.
Do you want just one particular value from one of the repeating sections? Is there a submit button in every repeated section?
Anyway, I am betting a button is inside the repeating section and when the button is pressed within a section you want to pull the value from the field.
I suggest using rules on the button to set fields in your datasource *outside* of the repeating group and those fields are used for the secondary data connection submit.
I've attached a sample for you to look at.