Marketable permits—also called tradable permits or transferable permits—differ from non-marketable ones in that they need not necessarily be used strictly for the purpose that they were granted; some scholars refer to ‘tradable entitlement systems’. If a firm does not use them fully, the unused part can be stored for the future or used by another firm. And vice versa: if a firm wishes to do more than its permit allows, the missing part can be purchased. Transfers may involve different entities. Some systems also allow transactions within a single firm; the unused permits can be used by the same firm in the future. Alternatively, a firm can deprive itself of a future use by using the permit earlier than it was originally granted for.