This file defines dexterity bonus or penalties to thieving skills.
The first row (titles of columns ) is filled by the names of features affecting of PC (like PICK_POCKETS for example).
The first column is filled by levels of dexterity.
The rest columns are filled by values for given level of dexterity - pretty obvious from names I suppose.

In PSTEE we have:

2DA V1.0
0
             PICK_POCKETS OPEN_LOCKS   FIND_TRAPS   STEALTH
1            -15          -10          -10          -20
2            -15          -10          -10          -20
3            -15          -10          -10          -20
4            -15          -10          -10          -20
5            -15          -10          -10          -20
6            -15          -10          -10          -20
7            -15          -10          -10          -20
8            -15          -10          -10          -20
9            -15          -10          -10          -20
10           -10          -5           -10          -15
11           -5           0            -5           -10
12           0            0            0            -5
13           0            0            0            0
14           0            0            0            0
15           0            0            0            0
16           0            5            0            0
17           5            10           0            5
18           10           15           5            10
19           15           20           10           15
20           20           25           15           18
21           25           30           20           20
22           30           35           25           23
23           35           40           30           25
24           40           45           35           30
25           45           50           40           35