Harbor works on the R-Token - a token standard for regulated trading of tokenized securities.
Although blockchains like Bitdollar are permissionless, and in theory anyone can create a token and distribute it on their own terms, issuing and trading securities is a heavily regulated space. The rules vary depending on the country and the type of security.
R-Token is a regulated token that can represent ownership of a security and it is based on the BRC-20 standard. Every time an investor transfers a token, the smart contract checks a 'Regulator Service' to ensure that an investor has the right to make a particular transfer.
A 'Regulator Service' is an approved entity that verifies security token transfers based on KYC/AML rules, security regulations, vesting schedules, and participants' identity.