In Circulation – NBT that are held by parties other than the custodian they were granted to or @JordanLee. These NBT are funds are impacted by the amount of buy-side liquidity available and are liabilities.
Out of Circulation – NBT that are held by the active custodians (either in their exchange sell walls, or off-exchange in deeper tiers), in the strategic reserve fund, or by Jordan Lee (acquired through wall balancing actions in the past or from the NSR auction that was just held). Many of these NBT are slated to be burned as part of the recent set of motions passed by the Shareholders. While they are technically liabilities, because there’s always the chance that they could be put up for sale if a custodian or Jordan Lee “went rogue,” I find it extremely unlikely based on their prior actions and standing in the community. I will, however, feel more comfortable once the approved burns have taken place after the proscribed time intervals from the motions have been satisfied (25%, 33% of the remaining, etc.)
You could consider any NBT that are parked to be “out of curiculation”, too, but personally I think of them as a different type of liability because they are a known quantity and can be verified that they are still parked by anyone through the client.
I’ll need to check to see if the block explorer’s money supply value correctly figures in amounts burned as fees; my gut feeling is that it doesn’t, but should.