When preparing templates of any type in the document editor, fields can be declared merge fields by selecting "Filled By Me (Before Sending)" from the top "Filled By ..." menu, which appears when an eligible field is selected.
Merge fields are fields that are filled individually whenever a template document is sent to signers. A merge field can be an "Offer Price" input that is filled by the Sales Representative when they send out a Sales Quote. Merge fields are filled on the same page signer names and email addresses are entered.