Sometimes, a picture is worth 1000 words so this illustration may help:
RelCol uses a Gamut Clipping (what Schewe likes to call the sex change operation). Perceptual uses a Gamut Compression. Neither is better or worse, the choice should be made after viewing the two and picking the process which produces the appearance you prefer.
The other very important key to understand is that there is no standard way to conduct a Perceptual rendering. Every profile making software can come up with a method to do this. Just like when viewing E6 film, Velvia looks different than Ektachrome. The manufacturers of those films, like the people who build Perceptual renderings produce what they think
their customers may prefer. So the quality or acceptability of a Perceptual rendering can vary. A lot! Non out of gamut colors can be affected with the Perceptual rendering because the idea is, alter the out of gamut colors to maintain a relationship with other colors so you perceptually deal with this process in a (better?) fashion.
There’s more to all this of course. For example, how a rendering intent treats an image that began in sRGB versus one that began as ProPhoto (by the time the rendering intent comes into play, the CMS has no idea which was which).
I’m wouldn’t dare ask Michael about pillow biting after that last post <g>!