The carbon is considered "custodied" because, if incoming reserves were to drop to 0 and the protocol simply paid out all remaining staking rewards, CC would be the final supply of KLIMA, backed at IV of 1 tonne of carbon offsets. So, in this endgame scenario, each 1 KLIMA would end up being a claim on 1 tonne of carbon offsets from the treasury - thus CC/KLIMA is the estimated number of carbon offset tonnes that a holder of 1 KLIMA today would have after all staking rewards are paid out if they remain staked until the end.