This file link the level of the given class (first column) with the Base Attack bonus (second colmun) and the number of attack (third column).
In IWDII we have in BAATFGT:

 2DA V1.0
 0
   	BASE_ATTACK	NUM_ATTACKS
1	1		1
2	2		1
3	3		1
4	4		1
5	5		1
6	6		2
7	7		2
8	8		2
9	9		2
10	10		2
11	11		3
12	12		3
13	13		3
14	14		3
15	15		3
16	16		4
17	17		4
18	18		4
19	19		4
20	20		4
21	21		4
22	22		4
23	23		4
24	24		4
25	25		4
26	26		4
27	27		4
28	28		4
29	29		4
30	30		4
31	31		4
32	32		4
33	33		4
34	34		4
35	35		4
36	36		4
37	37		4
38	38		4
39	39		4
40	40		4


It applies with the following files:

BAATFGT: fighter, ranger, paladin and barbarian
BAATMAG: mage and sorcerer
BAATMKU: monk fist attack
BAATNFG: bard, priest, druid, thief

It seems there are not one file per class.