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From the drop-down, select the type of join to apply. For more information, see Join Types. 

Join keys:

In the above image, the platform has determined that the item number (Item_Nbr) field of Region 1 data and the item number (ITEM_NBR)  field from REF_PROD should be used as the keys for performing the join.  

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To add the specified join to your recipe, click Add to Recipe.