An output intent describes the final destination device you will use to reproduce the color in the PDF, such as the separations printing device. Output intents can override working spaces during viewing and printing, but they do not convert the colors in the PDF.
There are two kinds of output intents: One includes an embedded device profile that defines the color space of the destination device, such as U.S. Web Coated (SWOP) v2; the other is a name that defines the destination color space and usually names a standard output condition. Using a named output intent rather than an embedded profile helps reduce the size of a PDF, but this is typically only possible for PDF/X-1a files, or PDF/X-3 files that do not contain color-managed color.
You include output intents when you create PDF/X (or PDF/A) files, using the Standards panel of the Adobe PDF Settings dialog box. (You access Adobe PDF Settings differently, depending on the application you’re using.) You can also use third-party plug-ins to include output intents. For documents with named output intents rather than embedded profiles, the program looks for the color profile associated with the named intent.