[ECI-EN] Things about color management that are not clear

Dear all, I work for a small packaging company whose activity consist of printing (offset), cutting and folding cardboard. Since not long, we have decided to handle PDF files from the client and produce the offset plates ourselves because we had too many problems with external providers. The change has been pretty quick and now that everything is +/- OK, we would like to improve greatly and manage color correctly. I have read many website as well as the “Digital Color Management” book from Jan-Peter Homann, but there are still a few things that remain unclear. It seems obvious that there are a few key elements that I haven’t understood and it will be of great help if you could shed some light on these matters. I have read everywhere that the PDF files sent by the clients should be CMYK that contain no ICC-profile. It still doesn’t make sense for me at the moment. I know I am wrong, but I don’t understand why. 1) When you set up a PDF document (eg. in Illustrator or CS Suite), you need to set up you working space. Mine for CMYK documents is defined on “ISO Coated v2 ECI”. What does it do actually ? Does it apply an ICC profile ? Does it transform colors ? I really don’t understand the mechanism ... 2) When you export your PDF (I know I should use PDF/X but I do need to keep transparency and separate layers), in the “Output” section. --> I need to choose “Convert to Destination (Preserve numbers)” --> Destination should be the same as working space (thus ISO coated v2 ECI) for correct color management --> ICC profile should not be included --> Output intent should be defined Is that right ? If it is, here is what I don’t get : - What is the difference between “Convert to Destination (Preserve numbers)” and “Convert to Destination”. In other words, what does the “Preserve numbers” option actually does ? - What would change if the ICC profile was included ? How will it inpede color management at the next steps ? I thought a document or image should always have an ICC profile associated so that software knew how to treat it ... - Does the output intent actually as an impact when opening the files in an application or is it just text information for the viewer that says how it should be printed ? - Do RGB images previously converted in Photoshop (let’s say for Iso coated) and then inserted in the PDF still contain ICC profile when the PDF is exported ? I am sure you have now realised that I still have not correctly understood color managmeent. This is why I am sending you this mail. Usually I would never do that, but I can’t find the answer anywhere else and I think it’s important to understand these concepts correctly. I hope some of you can help me Kind regards, Thibaut SIMONART