Re: "Please clarify if specimen is a MustSupport or "MaySupport", that is if a server NEVER sends Observation.specimen, does such server still conform to us-core-6?" Observation.specimen is a MustSupport element; the cited guidance intends to inform the reader that there are multiple ways to represent the specimen type in Observation and is not intended to imply that an implementer can support the US Core Laboratory Result Observation Profile and not support the US Core Specimen Profile. Edit bullet from The specimen type may be implicit  Observation.code  (e.g., Blood Glucose) or communicated using  Observation.specimen . In addition, the Specimen resource can share other information such as collection details.   to The specimen type can be communicated in the mandatory Observation.code (e.g., Blood Glucose), or the must support {{Observation.specimen }}element, or through both elements.