In an RDL, subclasses of 'person' will typically require 'role' restrictions to be considered fully explicit. For example, 'maintenance engineer' will be a subclass of 'person', as every engineer is a person. To capture what a maintenance engineer does, a role restriction can say that any member is the bearer of an engineering role, and what the role amounts to will be represented by the kinds of activities that count as realizations of the role.
