In general, there are two approaches to partitioning the types into subtypes: ( i) disjoint, mutually exclusive subtypes and ( ii) non-mutually exclusive types. For instance, the type PHYSICAL_OBJECT in Figure 4 is partitioned into the subtypes ROLE_TYPES and NATURAL_TYPES which are not mutually exclusive. The type NATURAL_TYPES, however, is partitioned into three domain-meaningful disjoint subtypes -- GAS, LIQUID and SOLID. The most common AI approach to building type hierarchies is to mix both kinds of partitioning because the exclusive classification at every level of the hierarchy is feasible only in limited domains. Some bigger AI hierarchies (e.g., Komet/Penman Upper Model [Bateman et al. 95]) often make mutually exclusive distinctions and contain respective (somehow dummy) type labels like String and Non-string. Building the classification, however, does not provide the distinction between the two types of partitioning without a respective explicit declaration. Such an explicit distinction between exclusive and non-exclusive types might not be needed in many applications but we found that it must be taken into consideration since it influences some generation decisions (see Section 6). Therefore, in [Angelova & Bontcheva 97], we proposed to label' all mutually exclusive types using the CHARacteristics relation. For instance, NATURAL_TYPES in Figure 4 has the characteristic EXCLUSIVE. If such a conceptual graph is missing, e.g., for PHYSICAL_OBJECT, by default this means that the subtypes of PHYSICAL_OBJECTS are not mutually exclusive. We decided to introduce this information complimentary to the type hierarchy, i.e., without redefining the ISA relation, since we wanted to be compatible with the basic theory [Sowa 84].