Output intent.

A declaration inside a PDF, not a conversion of its colours. Prepressure's PDF/X-4 page says such a file must carry an output intent describing the intended printing condition, either as an ICC profile, embedded or referenced, or as an output condition identifier such as FOGRA27. Gelato asks for PDF/X-4 with the output intent set to GRACoL 2006, and says X-4 supports both sRGB and CMYK objects. PDFengine stresses the label's limit: it tells the printer which press condition the file was prepared for but does not change a single colour value, so a file stamped with an output intent can still hold colour numbers that were never converted for that condition and be rejected for ink coverage. Setting the intent and converting the colours are separate steps in an export, and a platform that names an output intent is naming the first. Check the second yourself, against the profile the intent points to.