Unknown.
When the variabe in the 2nd column (headed "0") is set to 1 the area listed in the 3rd column (headed "1") is set visible and reachable on the worldmap.
In IWD2 we have:

2DA V1.0
*
                        0                       1
Targos_Docks            AR1000_REVEALED         AR1000
Targos                  AR1100_REVEALED         AR1100
Targos_Palisade         AR1200_REVEALED         AR1200
Shaerngarn_Ford         AR2000_REVEALED         AR2000
Shaerngarn_Ford2        AR2102_REVEALED         AR2102
Horde_Fortress          AR3000_REVEALED         AR3000
Western_Pass            AR4000_REVEALED         AR4000
Western_Pass_Icewall    AR4100_REVEALED         AR4100
Wandering_Hills         AR5000_REVEALED         AR5000
Cold_Marshes_Ent        AR5001_REVEALED         AR5001
Cold_Marshes_Exit       AR5004_REVEALED         AR5004
River_Caves_Ent         AR5005_REVEALED         AR5005
River_Caves_Exit        AR5102_REVEALED         AR5102
Black_Raven_Ent         AR5200_REVEALED         AR5200
Black_Raven_Exit        AR5203_REVEALED         AR5203
Underdark               AR5300_REVEALED         AR5300
Kuldahar                AR6001_REVEALED         AR6001
Dragons_Eye_Int         AR6100_REVEALED         AR6100
Dragons_Eye_Exit        AR6104_REVEALED         AR6104
Fields_Slaughter_Int    AR6200_REVEALED         AR6200
Fields_Slaughter_Exit   AR6201_REVEALED         AR6201
Severed_Hand            AR6300_REVEALED         AR6300