A proof is suitable for two types of color control: firstly, during the creation or retouching phase, e.g. to reconcile a color retouched image with the original, and secondly to check the final data directly before printing.
For control proofs during the data creation of a project, the data format usually does not matter. Whether PDF, JPEG, TIFF; EPS, PS or even PSD… Many proofing companies accept a variety of data formats. For a correct evaluation of the result, however, it is important to proof in the color space in which the print product is also created later. Data for a letterhead should therefore be proofed in ISOUncoated or PSOUncoated, while products printed on image printing paper should be proofed in ISOCoatedV2. For yellowish paper, newsprint or gravure printing, there are many other profiles for which a proof can be produced. You can find a good overview of the current proof profiles here. It is also important that the proof format and the final print format do not differ too much. Only in this way is a correct check possible.
When the brochure has been laid out or the catalogue production has been completed, a proof should be prepared again for the final check by the customer. This proof is then created with exactly the same data that is also sent to the print shop. This is usually a PDF X/3:2002 file, as this is the preferred data format for printers. If the pages are delivered to the printer with bleed marks and bleed, then the proofs should actually be created in exactly the same way. The finished proofs can then first be used as approval for the customer, and secondly for checking the OK sheet in the print shop. This ensures that no unpleasant surprises wait for the customer (what does the colour look like????) or the printer (why does the customer make a complaint?????) after printing and bookbinding.