This file would define which spells are given to priest and mage classes on level-up. With the default file, priests gain spells SPPR[1-9][**] and mages gain spells SPWI[1-9][*8] where ** is the column from this file. You can edit it so as to reduce the number of spells available below the first 50. Raising the number above 50 does nothing however.

In BG2EE we have:

2DA V1.0
0
            1       2       3       4       5   6   7
MAGE        50      50      50      50      50  50  50
PRIEST      50      50      50      50      50  50  50