selectionBehavior is CONDITIONAL; define CONDITIONAL

Fixes #509
and https://jira.hl7.org/browse/FHIR-28742

Author: isaacvetter

docs/specification/current.md

@@ -16,7 +16,7 @@ If a JSON attribute is defined with an optionality of OPTIONAL, but does not hav
 Unless otherwise specified, JSON string or URL (Uniform Resource Locator) attributes that have an optionality of REQUIRED MAY NOT have empty values (those without any characters or just whitespace characters).
 
 ### Conformance Language
-The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119](https://tools.ietf.org/html/rfc2119).
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this specification are to be interpreted as described in [RFC2119](https://tools.ietf.org/html/rfc2119). Further, the key word "CONDITIONAL" indicates that a particular item is either REQUIRED or OPTIONAL, based upon another item.
 
 ## CDS Hooks Anatomy
 
@@ -488,7 +488,7 @@ Field | Optionality | Type | Description
 `indicator` | REQUIRED | *string* | Urgency/importance of what this card conveys. Allowed values, in order of increasing urgency, are: `info`, `warning`, `critical`. The CDS Client MAY use this field to help make UI display decisions such as sort order or coloring.
 `source` | REQUIRED | *object* | Grouping structure for the **Source** of the information displayed on this card. The source should be the primary source of guidance for the decision support the card represents.
 <nobr>`suggestions`</nobr> | OPTIONAL | *array* of **Suggestions** | Allows a service to suggest a set of changes in the context of the current activity (e.g.  changing the dose of a medication currently being prescribed, for the `order-sign` activity). If suggestions are present, `selectionBehavior` MUST also be provided.
-`selectionBehavior` | OPTIONAL | *string* | Describes the intended selection behavior of the suggestions in the card. Allowed values are: `at-most-one`, indicating that the user may choose none or at most one of the suggestions;`any`, indicating that the end user may choose any number of suggestions including none of them and all of them. CDS Clients that do not understand the value MUST treat the card as an error.
+`selectionBehavior` | CONDITIONAL | *string* | Describes the intended selection behavior of the suggestions in the card. Allowed values are: `at-most-one`, indicating that the user may choose none or at most one of the suggestions;`any`, indicating that the end user may choose any number of suggestions including none of them and all of them. CDS Clients that do not understand the value MUST treat the card as an error.
 `overrideReasons` | OPTIONAL | *array* of **Coding** | Override reasons can be selected by the end user when overriding a card without taking the suggested recommendations. The CDS service MAY return a list of override reasons to the CDS client. The CDS client SHOULD present these reasons to the clinician when they dismiss a card. A CDS client MAY augment the override reasons presented to the user with its own reasons.
 `links` | OPTIONAL | *array* of **Links** | Allows a service to suggest a link to an app that the user might want to run for additional information or to help guide a decision.