Current Spoken Language Understanding models rely on either hand-written semantic grammars or flat attribute-value sequence labeling. In most cases, no relations between concepts are modeled, and both concepts and relations are domain-specific, making it difficult to expand or port the domain model. In contrast, we expand our previous work on a domain model based on an ontology where concepts follow the predicate-argument semantics and domain-independent classical relations are defined on such concepts. We conduct a thorough study on a spoken dialog corpus collected within a customer care problem-solving domain, and we evaluate the coverage and impact of the ontology for the interpretation, grounding and re-ranking of spoken language understanding interpretations.