What refers to a reference to a custom agreement that describes how the resource is being used (e.g. an implementation guide) that was followed when the resource was constructed, where the implementation guide must be known and understood in order to safely processing the content?

• A. Implicit Rules
• B. MustSupport
• C. Cardinality
• D. Version

Answer: (A)

Implicit Rules describe the external constraints or guidance that were followed to build the resource, such as an implementation guide (IG). The value is a URI pointing to that guide, informing processors how to interpret the resource and what rules apply beyond the base FHIR definitions. To safely process and understand the content, the recipient must reference and understand that implementation guide, since it specifies how the resource should be used and interpreted in that context.

This concept is distinct from mustSupport (which marks elements that must be supported by implementations), cardinality (the allowed number of occurrences of an element), and version (the resource or profile’s version).