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 BG2 ToB we have:

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