This paper presents the definition, reasoning and representation of part geometry characteristics to support manufacturability assessment. The scope is limited to the extraction of significant items formed from feature interactions for castability and moldability assessment. Reasoning for part geometry characteristics is performed as a refinement procedure. High-level spatial relationships between features - ``Is_In'' and ``Adjacent_To'' are first derived to support the specialization of the characteristics of individual features as well as other detailed relationships. The significant items formed from feature interactions are then extracted based on the detailed spatial relationships and geometric details.