What is a dieline in packaging?

Think of it as the box unfolded and laid flat. Solid lines are cuts. Dashed lines are creases, where the board is compressed so it folds cleanly instead of cracking. Shaded areas are glue flaps. Those three conventions are near-universal in the trade, and they are what a die-maker reads to build the physical tool.

The dieline is drawn before any artwork exists, because the artwork has to fit it rather than the other way round. It also has to account for the caliper of the board: a carton drawn for 300 gsm duplex will not fold correctly in 400 gsm, because thicker board needs wider creases and more generous tolerances at every corner. A template downloaded for one weight and used for another is one of the most common reasons a first sample does not close properly.

Cut, crease and perforation belong on separate layers in the file. They become separate rules in the steel die — a cutting rule, a creasing rule, a perforating rule — and a die-maker working from a single-layer drawing has to guess which is which.

Getting it wrong is the most expensive mistake in packaging, because the die is a physical steel tool that has to be remade. That is why the dieline is signed off at the design stage, before plates, and why a repeat order in the same size is cheaper than the first one: the tool already exists.