QFTP — notion of a constraint

I am struggling with the notion of a constraint, whereby operators (e.g. >, < and =) are used within an expression containing QoS characteristics and class attributes. I can see that QoSCharacteristic is a specialisation of QoSContext, although QoSDimensionSlot seems an odd place to declare operators. Should the model look like: Diagram Alternatively, I assume there is a pattern within the SysML parametric model.