ERC621 is an extension to the ERC20 standard. It adds two functions to increase and decrease the total amount of tokens in circulation. In short, it proposes that totalSupply can be changed. ERC20 only allows a single token emission event defined by the contract owner during creation. With ERC621 a new totalSupplycan be defined through the functions increaseSupply() anddecreaseSupply(). It is recommended that these functions are only accessed by the contract owners or trusted users. To enhance ERC621's functionality and security, and to avoid potential errors, additional functions for overflow checks, contract property modifications and restricted privileges, should be implemented.
Source: https://medium.com/coinmonks/anatomy-of-an-erc-an-exhaustive-survey-8bc1a323b541 Reference: ethereum/EIPs#621