One of the core 'tenets' of FHIR is that the resources are deliberately kept simple, but that there is a built in mechanism to allow them to be adjusted to meet a particular Use Case. At the level of the individual resource, this is done by:
Defining extra properties for a resource - called extensions. For example, a Condition does not (at the time of writing) have a property for acuity (whether a condition is acute or chronic). If we wanted to record this, we'd define an extension to do so.
Constraining existing resource properties. For example we might want to indicate that a particular property is never used in out Use Case - for example we don't support Patient.photo, or that Patient.identifier is required, single and must come from a specific Code System
(There is more that you can do, but the above is likely to be the more common ones)
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