This file defines how many proficiencies * a character will have at his/her creation, and how often he/she will get an other one.
First column is the list of classes identifiers.
2nd column is just the number of proficiency * given at the PC creation from the class defined in column 1st.
3rd column is the levels number that should be obtained to receive another proficiency *. A value of 0 causes a crash during character generation.
In BG2 ToB we have:

2DA V1.0
0
                        FIRST_LEVEL     RATE
MAGE                    1               6
FIGHTER                 4               3
CLERIC                  2               4
THIEF                   2               4
BARD                    2               4
PALADIN                 4               3
DRUID                   2               4
RANGER                  4               3
FIGHTER_MAGE            4               3
FIGHTER_CLERIC          4               3
FIGHTER_THIEF           4               3
FIGHTER_MAGE_THIEF      4               3
MAGE_THIEF              2               4
CLERIC_MAGE             2               4
CLERIC_THIEF            2               4
FIGHTER_DRUID           4               3
FIGHTER_MAGE_CLERIC     4               3
CLERIC_RANGER           4               3
SORCERER                1               6
MONK                    2               4