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 2-3 are modifications to the features described by their names (to hit bonus, damage bonus).
In IWD TotL we have:

2DA V1.0
0
           HIT        DAMAGE
0          0          0
1          0          0
2          1          2
3          3          3
4          3          4
5          3          5