Frequently Asked Question
The FHIR exchange specification is based on what is generally agreed to be common requirements across healthcare. However it is frequent and expected that many implementation will have more specific requirements that are not part of these agreed common requirements as incorporation of all possible requirements for every possible use worldwide into one specification would make it needlessly heavy and difficult to implement. Instead, FHIR expects that additional valid requirements not represented in the common basic resources will be implemented as FHIR extensions.
Every resource or datatype element may include one or more "extension" child elements to represent additional information that is not part of the basic definition of the resource/element. The extension is either simple or complex. Simple extensions have only a value and no nested extensions. Complex extensions contain one or more nested extensions and no value.
So it is normal on a country FHIR profile sometimes not to use an existing FHIR element and replace it with a national extension when the original FHIR element does not accommodate the local needs. This was done in Belgium with the AllergyIntolerance.Type element before the FHIR R5 modified this element from Codable Concept with a binding of "Required" toward increased flexibility of the value set with a binding of "Example". It is also normal to see extensions in national profiles that are meant to capture data that would not be captured in any element in the "basic" common international FHIR resource.