This file defines which spells are given to priest-types on level-up. Priests gain spells SPPR[1-9][**] where ** is the column from this file.
This file seems to have no effect on mages.
In IWD: TotL we have:

2DA V1.0
0
           1          2          3          4          5          6          7          8          9          
MAGE       19         23         19         24         19         20         11         8          3          
PRIEST     13         18         25         23         19         13         23         0          0