This file is not fully understood, but it is thought to control caching of biff files.

The first column the filenames of biff files.
The second column lists the number of valid entries (e.g. 2 means consult the AREARESREF1 and AREARESREF2 columns).
Subsequent columns list biff files to cache the the initial biff is loaded
In IWD2 we have:

2DA V1.0      
NO_AREA       
           NUMBER_AREAS AREARESREF1 AREARESREF2 AREARESREF3 AREARESREF4 AREARESREF5 AREARESREF6 
AR4103     1            AR4102      NO_AREA     NO_AREA     NO_AREA     NO_AREA     NO_AREA     
AR6104     1            ANMUNDD     NO_AREA     NO_AREA     NO_AREA     NO_AREA     NO_AREA     
AR6305     1            AR6303      NO_AREA     NO_AREA     NO_AREA     NO_AREA     NO_AREA