This subtype of the Assessment Event is used to bind the total score concept and total score meaning properties to the proper coded values for the PHQ-9, and to set the limits on the total score value to range from 0 to 27. Logically, we're saying that this Assessment Event is a PHQ-9 Assessment Event, although physically, there is little difference between this and other instrument-specific subclasses. In a database implementation, this would not result in a separate table - it would simply set the rules that dictate what the Assessment Event values may take. Not shown in this UML model is that the assessmentInstrument property must be a PHQ-9 instrument (LOINC code 44249-1). This could have been done by creating an association from this class to the PHQ-9 class using the same name (assessmentInstrument), thus redefining the parent property, but this technique can cause confusion and so the limitation on the type of assessment instrument is merely implied.