AbsSemRep I-el ~~ .................................... SemRep --(~------ Figure 3: A RAGS view of the CGS system In many NLG systems, (nominal) referring expression generation is an operation that is invoked at a relatively late stage, after the structure of individual sentences is fairly well specified (at least semantically).
referring expression generation needs to go right back to the original world model/knowledge base to select appropriate semantic content to realise a particular conceptual item as an NP (whereas all other content has been determined much earlier).