QUALIFIABLE is used in a reference property where one or more key components of the referenced object(s) need not be taken from other properties in the referring object, but may be supplied in the property itself. It is used when all key data cannot be stored in properties of a referencing definition, but the name of the referenced definition must be qualified by the key property.
For example, this KEYED BY clause:
KEYED BY {key_component-1: property_name_1, name}
states that the value of key_component_1 should be taken from property_name_1 and so the reference property would contain just the name(s) of the reference object(s). Whereas this KEYED BY clause:
KEYED BY {key_component-1 QUALIFIABLE, name}
states that the value of key_component_1 should be taken from this property – that is, the one with this KEYED BY clause, and so the reference property could contain the values of both the name(s) and the key_component_1(s) of the reference object(s). Under these conditions, the values of the names are separated from the values of the key-component_1(s) by periods.