This is what you see in the openings book tree:

__Moves__: (Tree icon) Sometimes a move will appear in grey, which means that it was never played, but leads to a position that results from some other line.

__N__: (N) The header displays the total number of games which led to the current board position. The N column displays the number of games in which each move was executed.

__%__: Percentage score achieved by the move, always from the point of view of the colour to play. If the number of games is low, the percentage is given in grey because it does not have great statistical significance.

__Av__: Elo average of the players of the move. If only a small percentage of the players had a rating, then the number is given in grey.

__Perf__: Elo performance of the move. This is the rating a player would have achieved in a fictitious tournament, playing the move in all his games.

__Fact__: The factor or "weights" allocated to each move control the probability with which it will choose the move in a game. The value can range between –125 to +125.The values are initially set to zero, but may change when the program actually plays games in the individual variations. You could say that the program is learning from experience. The weights can be manually modified by right-clicking a move and selecting “Change weight”. To reset all weights click Edit – Openings book – Reset weights. After that, the program will use only the purely statistical information to select its moves.

__Prob and [%]__: The first value is the purely statistical probability of the move,based on the number of times it was played and the results achieved in the games that went into the book. This is the probability of the move being played by the program as long as the weights have not been changed – either manually or by automatic learning. The value on the right [%] is the practical probability of the program playing the move. This is a product of the statistical evaluation and the weights. If you change the latter you can see how the [%] value (but not the “Prob”) changes. In Book options you can determine how much the weights influence the probability of play.