Indicates the criteron by which the reference range is valid. Different reference ranges may be defined based on the subject's age, race, sex, etc. "When the observation quantifies the amount of a toxic substance, then the upper limit of the range identifies the toxic limit. If the observation quantifies a drug, the lower limits identify the lower therapeutic bounds and the upper limits represent the upper therapeutic bounds above which toxic side effects are common." - HL7 Version 2.8, OBX-7 "Guidance on how to interpret the value by comparison to a normal or recommended range." - HL7 FHIR, Observation.referenceRange