This file assigns backstab modifiers to the according thief levels (I have no idea if it will change anything if somebody will add other class and give it backstab modifiers.)
In BG2 ToB 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         26         27         28         29         30         31         32         33         34         35         36         37         38         39         40         
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          5          5          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.