Currency
Items that change at least one property of an item. The affected item can be some other item. Currency items can also be traded. Here's a full overview of currency items.
A couple of items that belong to the class of currency items however do not change the effect of an item directly.
The Expedition Artifacts e.g. do not change an item directly. You can however use them to change items from the expedition vendors and they're necessary to use the The Recombinator. The Exotic Coinage is used to change the items provided by a certain expedidtion vendor in the Hideout.
The key's for example are used to change the state of chest in Trial Of Sekhema, so technically they're modifying a property (locked, to opened).
And last but not least Gold doe not provide any other values than spending it on the vendors and for reallocating Passive Skill Points and Atlas Skill Points.