This file defines the amount of experience points required to gain the next level by creature with class from first column.
In BG1 TotS we have:

2DA V1.0
-1
           1          2          3          4          5          6          7          8          9          10         11         12         13         14         15         16         17         18         19         20         
MAGE       0          2500       5000       10000      20000      40000      60000      90000      135000     250000     375000     750000     1125000    1500000    1875000    2250000    2625000    3000000    3375000    3750000
FIGHTER    0          2000       4000       8000       16000      32000      64000      125000     250000     500000     750000     1000000    1250000    1500000    1750000    2000000    2250000    2500000    2750000    3000000
PALADIN    0          2250       4500       9000       18000      36000      75000      150000     300000     600000     900000     1200000    1500000    1800000    2100000    2400000    2700000    3000000    3300000    3600000
RANGER     0          2250       4500       9000       18000      36000      75000      150000     300000     600000     900000     1200000    1500000    1800000    2100000    2400000    2700000    3000000    3300000    3600000
CLERIC     0          1500       3000       6000       13000      27500      55000      110000     225000     250000     675000     900000     1125000    1350000    1575000    1800000    2025000    2250000    2475000    2700000
DRUID      0          2000       4000       7500       12500      20000      35000      60000      90000      125000     200000     300000     750000     1500000    3000000    3500000    500000     1000000    1500000    2000000
THIEF      0          1250       2500       5000       10000      20000      40000      70000      110000     160000     220000     440000     660000     880000     1100000    1320000    1540000    1760000    1980000    2200000
BARD       0          1250       2500       5000       10000      20000      40000      70000      110000     160000     220000     440000     660000     880000     1100000    1320000    1540000    1760000    1980000    2200000

where:
Numbers in the first row of table determine level of creature.
Names in the first column are class identifiers. The rest columns have assigned xp values required to reach them by creature. One have to remember that it will affect only main classes, so entering kits identifiers will not give possibility to make separate level progression table for it.
Of course one could add more levels with defined experience values.