Data Fields are Data Elements added to a Data Model. Data Fields are the building blocks of a Data Model. A Data Field is a representation of a single piece of data targeted for extraction on a document.
For example, if you wanted to extract an invoice number from a collection of invoices, you would create an "Invoice Number" Data Field within a Data Model as its child. The extractor used to find that value on the document would be set on the Data Field, as well as other properties such as its ultimate format on output, how that field behaves during the Extract and Data Review activities (such as if the Data Field is required to be filled), and any .NET code based expressions used to validate or generate the field.