What defines external surface forms in dentistry?

The correct answer highlights the significance of external surface forms as a mold used in the fabrication of interim restorations. In dentistry, especially in prosthodontics, interim restorations are temporary solutions that maintain aesthetics and function while the final restorations are being prepared. A mold that defines the external surface forms is essential because it allows for the accurate replication of the tooth's contours and occlusal surfaces needed for these temporary restorations.

This process ensures that the interim restoration fits well, provides adequate occlusion, and meets the patient's aesthetic needs until the definitive prosthetic work is completed. A well-defined external surface form from a mold also contributes to the comfort and effectiveness of the interim restoration, making it a crucial aspect in the transitional phase of dental treatment.

The other choices refer to different aspects of dental impressions and preparations that do not directly pertain to the concept of external surface forms used for interim restorations.