A dieline is a flat structural reference for a package. It shows where the material is intended to be cut and folded and may identify glue or other production areas. Artwork is placed against it so the printed panels appear in the correct positions on the assembled pack.
Before designing against a dieline, ask which packaging structure and finished size it represents, who issued it, its units, its revision, and the meaning of its marks. Check panel orientation and keep important content clear of the confirmed production dieline’s cutting, folding, and glue areas according to the specific production requirements. A visual mockup or an old layout is not confirmation of the final production tooling.
For artwork handoff, see Packaging Artwork: Dielines, Bleed and DPI. The production dieline and manufacturing tooling still require confirmation by the responsible production team.