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The customer will be triggered to pull into a batch when a change has been made in the customer master or the contacts screen. Therefore the date and time in the Source Last Modified Date must be set to a date/time BEFORE a change was made in the customer master or the required fields were added to the Customer Master or Contacts screen.
Ex: If you added the required fields to Contacts fields on 8/1, you would want to set the date to 7/31.
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