Different types of Security Tokens are not regulated. However, they are nothing more than existing financial instruments or a mixture of them which have been around for a long time. Moreover, the legal framework for STO is the same as far as a usual security offering.
The modern twist of this, creating a security token (tokenization), means taking these existing financial instruments and starting to maintain shareholder balances or any rights that can be attached to a financial instrument on a distributed ledger or a blockchain by issuing digital tokens that present the shares of an underlying financial instrument on a blockchain. The main idea is to make these not only technically executable but also legally enforceable