In this part of Help we find
In some workflow scenarios, individuals submit filled-in forms as data-only files rather than as complete PDF files. These files are not PDFs, but use another file format, such as FDF or XML. You can view the data submitted by an individual recipient in the context of the PDF by opening the original file and importing the information in the data file.
In Acrobat DC , open the PDF form into which you want to import data. Choose Tools > Prepare Form . In the right hand pane, choose More > Clear Form .Note: When you import data from another file into a PDF form, the imported data replaces any information that appeared previously in the individual form fields. However, if the imported data file contains one or more blank form fields, importing will not clear the original data.
Choose More > Import Data .In the Select File Containing Form Data dialog box, select a format in File Of Type corresponding to the data file you want to import. Then locate and select that file, and click Open .
So obviously the Help documentation is not very consistent.
P.S. I tried looking for that Import Data feature in Acrobat DC, but was unable to find it.