Used by the Monster Summoning effect (#127), specified by the second parameter.
The first column is an identifer. The second column contains the filename of cre files. These are the creatures that can be summoned by the spell referencing this 2da.
The third and fourth column are unknown, they seem to link the animation played when creatures are summoned.
In IWD: TotL we have (asumm01.2da):

2DA V1.0
0
           RESREF     HitAnimation AreaHitAnimation 
1          AS1Bear    MSumm1H      ASumm1X          
2          AS1Wolf    MSumm1H      ASumm1X          

Description applies to: