Valuesets binding

The concept of valueset binding is extremely important in the FHIR specification. The FHIR standard defines different levels of recommendation/ obligation towards the use of valuesets. They are defined on the pages of HL7 but are partly copied here for easy reference.

Almost all the elements that have a coded data type are bound to a value set. The bindings are associated with various degrees of flexibility as to how closely the value set SHALL be followed:

required	To be conformant, the concept in this element SHALL be from the specified value set.
extensible	To be conformant, the concept in this element SHALL be from the specified value set if any of the codes within the value set can apply to the concept being communicated. If the value set does not cover the concept (based on human review), alternate codings (or, data type allowing, text) may be included instead.
preferred	Instances are encouraged to draw from the specified codes for interoperability purposes but are not required to do so to be considered conformant.
example	Instances are not expected or even encouraged to draw from the specified value set. The value set merely provides examples of the types of concepts intended to be included.

Important note: there can be differences concerning the precise conformance of ‘required’ and ‘extensible’ depending to which datatype they are applied. Consult the HL7 pages for extensive information.