Score Based Mode is very similar to DBWS ranking with the only difference that each war win here will earn your clan a score of current Difficulty value. Scores are being added up while you keep winning, but when you lose it is reset to zero - just like usual win streak. This system allows clans with higher difficulty to take high places without getting the longest win streak.
A current clan's score described above is called Win Streak Score (WSS) here. And the best such score ever achieved is Best Win Streak Score (BWSS) respectively.