GNU Backgammon reads several different files at start-up. It will first try to read a file called gnubg.wd. This is a file which contains all the neural net weights and is necessary for the program to evaluate positions. If this file is not found, or found corrupted or with an incompatible version, GNU Backgammon will search for a weights file called gnubg.weights. This file also contains the neural net weights, but is stored in ASCII format. If none of these files are found, GNU Backgammon will start, but it will give you a warning about these missing files, and it will play very poorly.
GNU Backgammon will also read the bearoff databases at start-up. There are usually two bearoff databases. The first is called gnubg_os0.bd, and has stored data about the bearoff probabilities of all position where both players have 15 or fewer checkers left on their six home board points. If this file is not found, GNU Backgammon will create a heuristic bearoff database for these positions. This database is a also called the 'One Sided Database', since it evaluates each player separately. The other bearoff database is called gnubg_ts0.bd. This bearoff database contains win probabilities for all bearoff positions with six or less checkers. It also contains cubeful equities (Money Game) for all the positions. If this file is not found at start-up, GNU Backgammon will evaluate late bearoff positions with the one sided bearoff database.