QTR-Quadtone RIP

OK...so I've got an R1900 and I've been using QTR on and off for a few months, and one thing has always puzzled me...

What purpose does the "Media Type" dropbox serve ("Photo Paper" or "Matte Paper")?

The choice to use PK or MK is made in the ink profile. Now I'll admit, I've been dutifully selecting the correct type depending on what I'm printing, but I started to wonder the other day if it actually made any difference.

I'm not at home for the next few days, so I can't try it out myself. So I thought I'd ask and see if someone else knows. Does it affect how the printer prints, and in what manner?

Thanks--

Greg