In many cases on a form you may be asking your applicants to type in the same answer several times. For instance you may ask for their full name in several places on the form but the answer will be the same each time.

With Existing Fields you can make it so that when they type the data in one field it will automatically populate into another field. 

You will find the Existing Field option in the bottom left of your SeamlessDoc (Uploaded PDF) 

After you click and drag and drop the field over onto the form it will then ask you to start typing in the label of the field you want it to pull the data from:
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Then after you click on the name of the field you want from the drop down menu, you're all set!