TODO: add hyperlinks for key terms to the relevant place in the FHIR spec for definitions/explanation
Note to designers: A modifer attribute which is missing leaves the resource unreliable, and potentially uninterpretable. So a modifier attribute SHOULD have either a default value or a minimum cardinality of 1. But we do not impose this on all resources as a SHALL; domain committees may be working with the reality that all existing information is unreliable - this is not uncommon. We do expect that most implementation guides - which can be more proscriptive - will nail this down.
What combinations of cardinality and '
IsModifier' are allowed?
Elements with mustUnderstand=Y should generally have a minimum cardinality of 1. If an element with mustUnderstand is omitted, it means that the parent element and all content within it can't necessarily be safely interpreted. No default value can be inferred. If it is possible for the value to be unknown, consider making minOccurs=1 and add an explicit coded value of "unknown" to make this explicit.
Default values are unsafe and interpretation can vary by context. Explicit values are safer
How do I interpret 'Must Support' with respect to slicing?
If an element is marked as 'Must Support' and is then sliced, that does not imply that each of the slices is 'Must Support'. If the intention is that a slice must be supported, then that slice should be marked as 'Must Support'. As an example, if Medication.identifier is marked as 'Must Support', and is then sliced to specify different types of identifiers, unless marked otherwise, the different slices are not 'Must Support'. This implies that a system must support Medication.identifier but can choose to support any of the slices, unless those slices were intentionally marked as 'Must Support'.
If an element is marked as 'Must Support' in its parent's slicing base, that does imply that the element is 'Must Support' in each of the slices. As an example, if Medication.identifier.system was marked as 'Must Support', and then Medication.identifier was sliced, the identifier.system would be 'Must Support' in each of those slices. NOTE: This means that if you do not intend an element to be 'Must Support' in every slice, that you should not mark it as 'Must Support' in the slicing base.
Resource element types
What are the criteria for selecting an appropriate data type?