I want to use Lr's Print module to prepare sRGB jpgs for upload to a print service (Mpix). Mpix only wants sRGB jpgs. For purposes of softproofing in another program (Capture NX2), I created an sRGB jpg from the Library Module, loaded it into NX2 and applied Mpix's downloaded printer/paper profile and compare it to the sRGB jpgs. It was clear that for this image relative colorimetric was a better choice than perceptual. That's all I used those jpgs for.
In Lr's Print Module, I set the "Print to:" dialog to "jpg file", set size and quality parameters and then under Color Management set "sRGB" and "relative colorimetric". As a test I also created a second jpg using "perceptual".
Both jpgs look identical and have identical histograms, at least as accurately as they can be displayed in Lr, yet the softproof showed clear differences. How does Lr embed the rendering intent into the jpg in way that the printer services's printer will understand? Does it set a flag in the EXIF? The help file for the print module says that the "print to jpg" is for the purpose of printing at an outside service but never really explains how the rendering intent affects the jpg files that it creates.