actually it's not a fancy lawyer game at all. it's a mathematical thing. it's based on the optimal way of distributing the reductions per country. the original idea was this: the world should reduce emission to reach pre 1990 levels, now, some countries haven't increased their emission that much since then, so, instead of everyone reducing their emissions the same relative amount you "trade" how much emission reduction you do with other countries, this achieves reducing the overall cost of the whole thing.