This is a common question from designers setting up files that use specialty inks in a digital printing environment. This following process works for vector-based design software such as Illustrator or InDesign (there are changes when using Photoshop or other raster-based programs). When creating artwork, it's best to separate the specialty ink by creating a custom layer named "special ink." You'll also want to create a custom spot color swatch and call it by the same name. Ensure that the spot color is noticeably different from the base artwork. Treat the specialty ink layer and spot specialty ink color as its own artwork and place any design elements requiring the specialty ink on this layer. Ensure that no other layers use the specialty ink color. When you export the file to a Press Ready PDF in InDesign, open the output section in the PDF export screen, select Ink Manager, and ensure that the spot color "special ink" is exporting as a spot and not process color. Illustrator will automatically save the spot color to a PDF when you Save As PDF.