This file assigns backstab modifiers to the according thief levels.
In BG1 TotS we have:

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

The first row contains of levels of the PC/NPC
The second row contains of the values of backstab modifier.
Changing the backstab modifier for level 0 affects all classes. The backstab modifier can be raised to at least x60 (although in-game feedback text only goes up to a x9 modifier).
This file cannot be extended with new classes or kits.