This file defines the  modification of weapon speed, damages inflicted and to hit bonus, based on weapon proficiencies.
where:
first column simply is the number of stars in the given proficiency
The columns 2-4 are modifications to the features described by their names (to hit bonus, damage bonus , speed bonus)
In IWD2 we have:

2DA V1.0
0
        HIT     DAMAGE
0       -4      0
1       0       0
2       1       0
3       1       2