|A Flat files says nothing about the format except that it isn't part of a relational database.|
A flat file is a stand alone data file that exists on its own without any reference to any other data files.
Excel, CSV and plain text are all flat files as they do not have any relational abilities. A batch file is only capable of generating a flat file as you need a relation database manager to manipulate relational files.
Maybe they are asking for a flat file without understanding what the term means.