The class 'CategoryConstraint' enables a category-based constraint to be specified. For example, in aviation the only clouds of significance for terminal aerodrome operations are convective clouds (cumulonimbus, towering cumulus etc.). The observed physical property 'cloud base [height]' may be constrained such that it is applicable only to clouds of a given type. In this example, a single instance of CategoryConstraint would be defined referencing both cloud types (cumulonimbus and towering cumulus); 'cloud type' is the constraint property.