A decal is a printed marking — unit numbers, caution stencils, insignia — applied to a model’s surface. The three families are waterslide (soaked and slid off backing paper), dry transfer (rubbed on), and sticker (peel-and-stick).
Markings sell scale: a field of tiny crisp warnings makes a hand-sized model read as a machine with a maintenance manual. Waterslides are the modeler’s standard because the carrier film can disappear entirely under setting solution and a topcoat.
See it in context in our panel lining & decals guides, or explore the tools hands-on in The Bench.