This case-insensitive code system http://interopehrate.eu/fhir/CodeSystem/Function-Codes-IEHR defines the following codes:

Code	Display	Definition
ageFromDate	AgeFromDate	The Function returns the age of the patient at the time the resource was created. The age is computed from the birthdate of the patient and the creation date of the resource
count	Count	The Function returns the number of matching resources, but not the resources
data	Data	The Function removes the text element from the returned resource
summary	Summary	The Function returns only the attributes of the resource that are marked as summary in the resources StructureDefinition
element	Element	The Function returns only the attributes of the resource that are specified in the parameters