A Selector which describes the selection of this Resource Selection within its source.

A Resource Selection may have multiple Selectors; in this case, the Selectors are treated as broadly equivalent (understanding that they may have subtly different resolutions or semantics); an application may choose freely between them. To take the intersection of multiple Selectors, one must be refined by the other. When a Selector only applies to a particular State, the State must be refined by that Selector.