What is a proof?
A proof (correct: contract proof) is an ISO-certified testing device for the graphic arts industry. A proof simulates the colourfulness of offset or gravure printing in a colour and legally binding manner within the narrow tolerances of ISO 12647-7. Today, it is almost exclusively calculated using a RIP and then produced with inkjet printers on special proof papers.
The proof data is converted into separations, then reassembled into a composite image to correctly simulate overprinting and trapping. The data is then transferred as a newly created composite image to an inkjet printer, usually with more than 8 colours, which prints the data. In addition to the print data, a proof must also carry a UGRA/Fogra media wedge in order to be colour-consistent and legally binding. Thanks to the standardised wedge, the printer is able to check the proof for correctness. Since many printing companies do not have this measuring technique at hand, the proof is often provided directly with a test report that shows the correctness of the measured values of the media wedge directly on the proof.
Earlier methods such as Chromalin etc. are no longer available on the market today.
ISO 12647 defines the term “Validation Print” (ISO 12647-8) in addition to the highest standard of contract proof, “Proof” (ISO 12647-7). The Validation Print is characterized by the fact that although it is less accurate in colour, it can also be produced on laser printers. Compared to the contract proof, however, it accepts much higher colour deviations and is only legally binding after prior consultation. A real “proof”, i.e. a real contract proof according to ISO 12647-7 is currently not only by far the best variant in terms of colour, but also the only legally binding proof.
Further information: https://en.wikipedia.org/wiki/Prepress_proofing