There are a number of different security token standards in the market, here's a comparison of the most popular frameworks.
Most of the token standard created by extending ERC20 standard so that token created using these standards should be backward compatible.
Backward compatibility will help Security token to integrate easily with existing eco system created around ERC20 standard and with the least of efforts and this is the same reason why many leading ST standards like ST20, T-REX, R Token, DS token etc are created in a similar method.
Source : tokeny