This file defines dexterity influence on characters abilities like success in spell learning or max possible maximal spells level that can be obtained by creature.
The first row (titles of columns ) is filled by the names of features affecting of PC (like LEARN_SPELL for example).
The first column is filled by value of dexterity of character.
The rest columns are filled by values for given dexterity - pretty obvious from names I suppose.
In BG1 TotS we have:

 2DA V1.0
 0
           REACTION   MISSILE    AC         
0          -20        -20        5          
1          -6         -6         5          
2          -4         -4         4          
3          -3         -3         3          
4          -2         -2         2          
5          -1         -1         1          
6          0          0          0          
7          0          0          0          
8          0          0          0          
9          0          0          0          
10         0          0          0          
11         0          0          0          
12         0          0          0          
13         0          0          0          
14         0          0          0          
15         0          0          -1         
16         1          1          -2         
17         2          2          -3         
18         2          2          -4         
19         3          3          -4         
20         3          3          -4         
21         4          4          -5         
22         4          4          -5         
23         4          4          -5         
24         5          5          -6         
25         5          5          -6