Nearly all decking manufacturers make both grooved and non-grooved boards. Grooved boards were designed to accept a special hidden fastener that slips into the clip and is then secured into the deck joist.
Grooved decking is generally only used on "field boards" in the middle area of a deck. Anywhere the board edge is visible, it's usually recommended to use a non-grooved board for aesthetic reasons.
If you don't have enough non-grooved boards and have leftover grooved ones, they can be used strategically in spots like the back stair tread, the inner double border board, or other spots where the edges don't matter.
Non grooved boards may also be called square-edged or bullnose boards, so if you see these terms used interchangeably.
Square-edged boards are used in applications where you will need to face-fasten with a screw, or when the board edge is visible. This usually includes borders/picture frames, stair treads, post wrap collars, and drink rail boards. They can't be fastened with hidden fastening clips, but can still be secured with a screw-and-plug fastener to achieve a nearly-invisible look that keeps the fastener head below the deck surface. If you're laying out a grooved board next to a square-edge, you have two options. The first and easiest option is to secure one edge of the grooved board with a hidden fastener clip (like in the photo) and then use screws on the edge next to the non-grooved board. Your second option is to route a groove in one side of the square-edge board and use a hidden fastener clip on the grooved side of it, and a screw on the other
Most manufacturers make grooved 12', 16', and 20' boards, but may not make square-edged boards in all the same lengths. Since many decks may have a 12' field area and need a 13' border board, you'd have to upsize your border board length to a 16' and wouldn't order a 12' square-edge board. Some manufacturers have the philosophy that since it's only on or two boards on your border, that even 16' non-grooved boards are not needed, so they only sell 20 foot non-grooved boards.
