Choice column

Provides guidelines for rendering multi-column choices. I.e. when displaying a list of codes (for `choice` or `open-choice` items) or a list of resources (for `reference` items), this extension allows the drop-down to have multiple columns. For codes, the author can pick additional code system properties to display - such as alternate display names strength or form for drug codes, etc. For references, the author can choose particular columns from the resource (e.g. first name, last name, specialty, address).