"Common Good" parachains are parachain slots reserved for functionality that benefits the ecosystem as a whole. By allocating a subset of parachain slots to common good chains, the entire network can realize the benefit of valuable parachains that would otherwise be underfunded due to the free-rider problem. They are not allocated via the parachain auction process, but by the on-chain governance system. Generally, a common good parachain's lease would not expire; it would only be removed via governance.
The purpose of these parachains will probably fall into one of two categories: system level chains or public utility chains.
System level chains move functionality from the Relay Chain into parachains, minimizing the administrative use of the Relay Chain. For example, a governance parachain could move all the governance processes from the Relay Chain into a parachain. Adding a system level chain is generally uncontroversial, because they merely move functionality that the stakeholders already agreed was useful from one place (the Relay Chain) to another (a parachain).
Moving the logic from the Relay Chain to a parachain is an optimization that makes the entire network more efficient. Moving system level logic to a parachain frees capacity in the Relay Chain for its primary function: validating parachains. Adding a system level chain could make the network capable of processing several more parachains. Rather than taking a slice of a 100 parachain pie, a system level chain takes one slice and bakes a bigger pie.
The vast majority of common good chains will likely be the unopinionated system level chains.
Public utility chains add functionality that doesn’t exist yet, but that the stakeholders believe will add value to the entire network. Because public utility chains add new functionality, there is a subjective component to their addition: the stakeholders of the network must believe that it is worth allocating a slot that would otherwise go to the winners of an auction, and thus would have an objective expression of conviction from its backers. Governance provides the means to internalize the value of the parachain slot and distribute it across all members of the network.
Public utility chains will always be fully aligned with their Relay Chain stakeholder base. This means that they will adopt the Relay Chain's native token (i.e. DOT or KSM) as their native token and respect any messages incoming from the Relay Chain and system level parachains at face value.
Some examples of potential public utility chains are bridges, DOT/KSM-denominated smart contract platforms, and generic asset chains.
Public utility parachains would typically grant privileged business logic to Polkadot’s governance. Just as the Polkadot Relay Chain has several privileged functions like setting the validator count or allocating DOT from the Treasury, these parachains can have privileged functions like changing system parameters or triggering an upgrade.
Because public utility chains add functionality beyond the scope of the Relay Chain, they will likely be approved by the network stakeholders only in rare scenarios.
Statemint (and its cousin Statemine on Kusama) will likely be one of the first common good parachains. Statemint is a public utility chain in that it adds functionality not available in the Relay Chain, namely the creation and management of assets.
Statemint will support both funglible and non-fungible assets. The chain offers an interface similar to ERC-20 for fungible assets and ERC-721 for non-fungible tokens. These interfaces are in the logic of the chain itself; by encoding this logic directly into the Statemint runtime, token storage and actions do not need to be metered and can happen faster and cheaper.
Like most common good chains, Statemint will use the DOT token as its native token, i.e. represented in its instance of the Balances pallet. Statemint trusts messages about balances from the Relay Chain, and vice versa, so users can transfer DOT from the Relay Chain to their address on Statemint and back.
Because of the efficiency of executing logic in a parachain, the transaction fees and deposits (including the existential deposit) are about 1/10 of their value on the Relay Chain. These low fee levels mean that Statemint is well suited to handling DOT balances and transfers as well as managing on-chain assets.
As a common good parachain, Statemint must stay fully aligned with the Relay Chain. Upgrades to Statemint will require the Relay Chain's "root origin", i.e. a referendum. Some of the other logic (like privileged asset functionality) will defer to the Relay Chain's Council.
See the Bridges page for information on the latest bridge projects.