Knockout protects the colour of the object on top.
Most coloured objects knock out by default. The lower separations leave an opening and the upper object prints into that space. A yellow logo over a blue panel therefore keeps its own ink build instead of mixing with the blue beneath it. Knockout is often the expected behaviour for opaque-looking coloured artwork, photographs, reversed lettering and shapes whose colour must remain distinct.
The opening and the object still have to align on press. Where adjacent colours meet, production trapping may create a tiny controlled overlap to reduce the chance of a paper-coloured hairline. Trapping is tied to inks, substrate and output method. Do not invent a universal manual stroke around every object; ask how the receiving workflow handles it.
Overprint keeps the inks underneath.
An overprinting object does not cut its shape out of the lower separations, so those separations remain present in the overlap. The printed result then depends on the inks, substrate and press sequence. This is useful for selected small black text and fine black rules, because a slight registration movement is less likely to reveal a white halo. It can also create an intentional mixed colour when one transparent process or spot ink prints over another.
That mixture is the danger as well as the purpose. Magenta over cyan will not remain plain magenta. A dark object over an image can allow details and colour to influence the result. Compare the two uses in the rich black and 100% K guide: a broad panel and a line of small type may need different treatment.
White set to overprint can vanish.
In ordinary process printing, the white seen in a layout usually represents the absence of ink and lets the paper show. If a white process object is set to overprint, it contributes no ink and does not remove the colour below. A reversed word or white logo may disappear in output even though it remains visible in a standard preview.
Preflight tools commonly warn about white overprint, but the warning needs context. A separate white-ink plate is different: it represents a real special ink and may be intended to overprint or underprint another layer. A real white-ink plate follows the construction described in the white ink underbase guide. Check the actual swatch type and separation before changing the attribute.
Placed files and effects can carry hidden attributes.
An Illustrator logo, supplied advert or placed PDF may already contain overprint settings. Changing the frame in the page-layout file does not necessarily alter objects inside the placed artwork. Transparency, blending modes and spot colours can also affect the final separations. A composite preview alone can make these interactions look harmless.
Open the original application when an attribute needs correction, then update the placed file and export again. Avoid flattening, rasterising or converting a whole page merely to remove one uncertain setting. Confirm the appropriate transparency handling against the PDF/X requirements for the job.
Special finishes need explicit separation and overlap instructions.
A varnish, foil mask, white layer or other finishing plate is usually represented by a named spot colour. Its on-screen colour is only a visual label. Whether it should overprint the CMYK artwork, knock out an area or sit in a separate file depends on the production specification. Converting all spots to process can destroy that instruction.
Keep each effect on a clearly named layer or separation and state the intended physical result. A spot UV patch over a photograph, a foil title on unprinted stock and a white underbase beneath coloured ink are three different overlap relationships. Use the foil, embossing and spot UV artwork guide for the handover details.
Turn off plates to reveal what the composite hides.
Open the exported PDF in Output Preview or an equivalent production tool. Enable overprint simulation, then isolate each process and spot separation. Turn off black to see whether small black text has knocked out unexpectedly. Turn off a finishing plate to confirm that it contains only the intended mask. Examine white lettering over colour and any object designed to hide artwork beneath it.
The preview predicts separation logic; it does not reproduce every physical ink, trap, material or press condition. Record any deliberate exception and include it in the proof discussion. Use the proof and colour options to state whether the next approval concerns content, separation behaviour, colour or the finished effect.