Uniqueness Database File, or UDF, is a text file that supplements an answer file during unattended installations of Microsoft software such as Windows NT and Windows 2000. Uniqueness Database Files (UDFs) override the general answers that the answer file provides to prompts during unattended setup with information that is specific to a given computer or small group of computers.
UDFs typically supply information such as computer name or username for each installed computer, while answer files specify more general setup options such as Typical setup or time zone, which are the same for all computers on the network. Thus, while you might need only one answer file for all the computers on which you are installing Windows NT in your local area network (LAN) or at your geographical locale, you need a separate UDF for each computer or small group of computers.
The UDF is merged into the answer file at the start of the graphical user interface (GUI) portion of setup, so you can use UDFs only to define setup parameters for the GUI portion of setup, not the text portion. During the text portion of setup, only the answer file can specify parameters to use in response to setup prompts.