A 'cupping' defect of the occlusal/incisal surfaces of teeth, with exposed dentin in a saucer-shaped depression, typically results from a combination of which two processes?

Cupping defects on occlusal/incisal tooth surfaces classically result from a combination of erosion (chemical dissolution of enamel/dentin, often from dietary acids or gastric reflux) and attrition (mechanical tooth-to-tooth wear) acting together — the chemically softened dentin from erosion wears away faster under the mechanical forces of attrition, creating the characteristic cupped/saucer-shaped depression.