The Bank of Internal Settlements (BIS) wants to start marking bitcoins. The central bank of central banks is essentially proposing to create a grading system from 0 to 100 for each satoshi, based on transaction history. If the score is low, companies and even users could refuse to accept that bitcoin, claiming it would be "tainted" by links to illicit activities. If this were implemented globally, it could create a parallel market where "clean" bitcoins would have different values than "marked" bitcoins.