Concise answer
Under-eye hollows are depressions or shadowed transitions, while eye bags are outward bulges or puffiness. Both can coexist, and dark colour may make either look more pronounced.
Bone structure, fat distribution, skin quality, fluid and lighting all shape the under-eye appearance.
Treating a shadow as pigment or a bag as a hollow can produce an unsuitable plan.
When evaluation may help
In-person assessment is important before injectables or energy procedures, and sudden one-sided swelling or visual symptoms need medical attention.