The premium (fka subsidy) is proposed in FIPs#243 (July 2021)
The first FIP draft is in 
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Abstract
The Filecoin network is secured by continually proven storage capacity, for which providers (miners) earn block reward. To incentivise the storage of useful data, the Filecoin Plus program incentivises the storage of verified deals by causing them to count for 10 times the storage power as other sectors, thus earning 10 times the block reward. This requires on-chain metadata about every sector's composition, which is expensive and prevents efficient aggregate accounting. It also couples network security to incentive distribution policy, weakening the former. The coupling greatly constrains the design space for more scalable mechanisms as capacity and deal utilisation increase. It will also prevent the effective development of alternative storage markets once the FVM enables user-programmable contracts.
The proposal breaks the coupling by rewarding Filecoin Plus verified deals explicitly, rather than piggybacking on the storage power block reward. In brief:
- Remove per-sector on-chain sector quality information from state; use raw-byte power as the only notion of consensus power.
- Account for verified deal space-time directly in the market actor.
- Split the block reward into consensus reward and verified deal subsidy, paid separately.
- Add pledge and penalty mechanisms to verified deals to retain the existing economic incentives.
See also
- Notes on economic implications and calculation of pledge function (BlockScience. Note: incorrect assumptions)
- Notes on reward distribution function (note: incorrect)