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When concepts are required to be grouped then we use concept set. For eg: Blood is a sample and many lab tests can be performed with it. So, we use concept set for Blood and define lab tests as its members. The concept set is nothing but a concept with specific property enabled (Is Set?). When set is enabled, we can add members to it. The member should be another concept. We should not include another concept set as member. You can find more info at Concept Dictionary Basics.
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