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Semantic interoperability refers to the ability of separate systems to understand each other. Semantic interoperability is build upon syntactic interoperability. HSPC achieves semantic interoperability using FHIR Profiles. An example of semantic interoperability is where two systems agree that a "patient" with a coded value for "ethnicity" will use the HL7 codes for ethnicity (http://www.hl7.org/fhir/v3/Ethnicity/index.html). There is a lot of modeling activity around different profiling efforts. Profiles that are known to the HL7 FHIR community can be found on the Profiles tab of the Resource definition (ex: http://www.hl7.org/fhir/observation-profiles.html).
Detailed Clinical Models
From wiki.hl7.org: Detailed Clinical Models (DCM) are descriptions of items of clinical information that include the clinical knowledge on the concept, the data specification, a model and where possible, technical implementation specifications. A DCM is a conceptual specification of the semantics of discrete structured clinical information. It provides the data elements and attributes, including the possible values and types of the attributes, needed to convey the clinical reality in a fashion that is understandable to both clinical domain experts and modelers.