It is a one-time-use token with a limited amount of data (somewhere between 0 and 64 bits from the implementations I've heard about). You go through a process

It is a one-time-use token with a limited amount of data (somewhere between 0 and 64 bits from the implementations I've heard about). You go through a process to get either one token or a 'batch' of tokens issued, then they are one-time-use redeemed to indicate authorization. Because they use a blind signature method, the server which issued them can't recognize them when they are redeemed. There is also a zero-knowledge proof used to make sure the server isn't trying to get additional bits of state by using different public keys per user. So they are not meant to track a user, but rather indicate that some process happened - that you previously solved a captcha, that you saw an ad, etc. 64 bits is way too much information IMHO, but the implementations are still very early.