What is a proof in printing?

A digital soft proof is a PDF. It is free, immediate, and the right tool for confirming content: spelling, layout, the position of a barcode, whether the artwork sits correctly on the dieline. What it categorically is not is a colour proof. A monitor emits light and a press lays ink on board; the two cannot be made equivalent, and approving colour from a screen is how disputes start.

A printed proof moves onto physical substrate. Produced digitally on the production stock it gets close to the final appearance, though colour remains indicative because a digital press is not an offset press. Produced as a full offset pre-production sample — with the actual plates, inks, substrate and finishing — it is genuinely accurate, which is why it carries a real cost.

Approval is the point that matters. Signing a proof is the moment the job becomes fixed: after it, changes mean new plates, a new make-ready and a new run. Being explicit about that is more useful to a buyer than being relaxed about it.

The approved proof is then the reference for everything downstream. In-process checks compare press sheets against it, and it is retained on file so that a later query is settled against what was actually signed.