Interface for retrieving all possible and settable values.
While possible values specify which values existing data might have, settable values define the values that are allowed to be set by a user.
For example, in an workflow scenario, the settable values for a state field might depend on the currently set state, while possible values are all states. Thus settable values would be used in an editing context, while possible values would be used for presenting filtering options in a search.
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