Files with undf file extension can be typically found as files in some kind of Undefined File Format. Most likely highly specialized.
UNDF file extension - Undefined file format
What is undf file? How to open undf files?
File type specification:
File extension undf is sometimes used by various recovery software for files that were recovered, but their format could not be recognized.
It can't be easily described what kind of file type it is. The most common software that mentions this file type is VLC, but it's just a generic label for undefined file type.
