Specimen Vs SpecimenDefinition

I am very new to FHIR so bear with me. I am looking at the Specimen and SpecimenDefinition. Is there an overlap here in data or am I just not getting it?

SpecimenDefinition is used with ObservationDefinition when defining lab masterfiles talking about the “type” of specimen required for a test (and possibly also indicating what additives or pre-processing of the specimen are needed). Specimen is talking about a specific collected specimen for a specific patient and is used with the Observation resource (which deals with a specific result for a specific patient). So one’s talking about something that can exist and the other’s talking about what does exist.


Oh, I see! The combination of SpecimenDefinition and Observation Definition looks promising.
Thanks alot!