Complex sentence: Thus "BIS" represents the Bismarck Archipelago at Level 3; as this area is not subdivided, "BIS-OO" can be used to represent it at Level 4.
Split into simpler sentences: Thus "BIS" represents the Bismarck Archipelago at Level 3. This area is not subdivided, so "BIS-OO" can be used to represent it at Level 4.