The file extension UDC identifies so-called user-dictionaries, ie user dictionaries of the DTP (Desktop Publishing) solution Adobe InDesign. With InDesign, for example, you can create print templates and layouts for print products such as flyers, catalogs or brochures.
UDC files can be created in InDesign via 'Edit-> Preferences-> Dictionary'. They contain added words as well as redefined words of the standard dictionary. You can also import UDC files from older InDesign versions into this menu.