This file defines the amount of experience points required to gain the next level by creature with class from first column.
In IWD TotL 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         21         22         23         24         25         26         27         28         29         30         31
MAGE       0          2500       5000       10000      20000      40000      60000      90000      135000     250000     375000     750000     1125000    1500000    1875000    2250000    2625000    3000000    3375000    3750000    4125000    4500000    4875000    5250000    5625000    6000000    6375000    6750000    7125000    7500000    8100000
FIGHTER    0          2000       4000       8000       16000      32000      64000      125000     250000     500000     750000     1000000    1250000    1500000    1750000    2000000    2250000    2500000    2750000    3000000    3250000    3500000    3750000    4000000    4250000    4500000    4750000    5000000    5250000    5500000    8100000
PALADIN    0          2250       4500       9000       18000      36000      75000      150000     300000     600000     900000     1200000    1500000    1800000    2100000    2400000    2700000    3000000    3300000    3600000    3900000    4200000    4500000    4800000    5100000    5400000    5700000    6000000    6300000    6600000    8100000
RANGER     0          2250       4500       9000       18000      36000      75000      150000     300000     600000     900000     1200000    1500000    1800000    2100000    2400000    2700000    3000000    3300000    3600000    3900000    4200000    4500000    4800000    5100000    5400000    5700000    6000000    6300000    6600000    8100000
CLERIC     0          1500       3000       6000       13000      27500      55000      110000     225000     450000     675000     900000     1125000    1350000    1575000    1800000    2025000    2250000    2475000    2700000    2925000    3150000    3375000    3600000    3825000    4050000    4275000    4500000    4725000    4950000    8100000
DRUID      0          2000       4000       7500       12500      20000      35000      60000      90000      125000     200000     300000     750000     1500000    1800000    2025000    2250000    2475000    2700000    2925000    3150000    3375000    3600000    3825000    4050000    4275000    4500000    4725000    4950000    5175000    8100000
THIEF      0          1250       2500       5000       10000      20000      40000      70000      110000     160000     220000     440000     660000     880000     1100000    1320000    1540000    1760000    1980000    2200000    2420000    2640000    2860000    3080000    3300000    3520000    3740000    3960000    4180000    4400000    8100000
BARD       0          1250       2500       5000       10000      20000      40000      70000      110000     160000     220000     440000     660000     880000     1100000    1320000    1540000    1760000    1980000    2200000    2420000    2640000    2860000    3080000    3300000    3520000    3740000    3960000    4180000    4400000    8100000

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.