"One piece" refers simply to a decal that is finished as a single piece - rather than a custom silhouette cut around individual components of the design that would turn each decal into multiple pieces.
One piece decals are sometimes larger in size involving more complex graphics or custom shapes. Decals on RV's or add on equipment such as canopies, trailers and spare tire cases are sometimes referred to as one piece decals. They act as a dominant branding tool for any add on or aftermarket equipment.
One piece decals can be domed for a more durable and premium finish. Custom doming on large decals makes them more substantial than flat vinyl decals making domed decals suitable for even the toughest marine applications. Doming also provides the decal with a glossy, 3d appearance.
Whether your design calls for a large or small decal, one piece decals are often the most cost effective often in the dealership decal line up.