In this context, a "set" usually refers to a specific photoshoot session, often featuring:
Rumor, passed down through obscure 3D modeling forums and forgotten IRC logs, tells of a digital artist from the early 2000s who signed their work only as "Willey." Willey was obsessed with impossible geometry: shapes that could exist in 3D space but not in the physical world. Non-orientable surfaces. Klein bottles of the soul. Willey Models Chelda Model - 3 sets -.zip.rar