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.
In BG1 TotS we have:
2DA V1.0
1          wolfsu     
2          dogwisu    
3          dogwasu    
Description applies to:
anisum01.2da - Animal Summoning I
anisum02.2da - Animal Summoning II
monsum1.2da - Monster Summoning I
monsum2.2da - Monster Summoning II
monsum3.2da - Monster Summoning III