Tr3v0r07

Twenty-three cohorts followed behind, that came from the contributions of the King of Makabakam, for though the bond between man and spren was at times inexplicable, the ability for bonded spren to manifest in our world rather than their own grew stronger through the course of the oaths given. - From Words of Radiance, chapter 35, page 9 I believe that Shardplate is a further manifestation of the bonded spren, allowed by stronger oaths, given by the knights, as insinuated by the quote above. Because the plate(post Recreance) is not the primary manifestation it cannot be bonded by the wearer as a Shardblade is, yet if feeds off of the stormlight, in either gemstones or invested by the wearer. This, and the oaths giving the Knights a higher effeciency in stormlight use and investing, could explain why the helmet worn on Kalidan's hand during the duel, drained his stormlight so quickly. He had, at this time only sworn 2 of the 5 ideals. This leads me to believe that Shardplate is made manifest by the bonded spren by speaking either the fourth or fifth ideal, all based on the particular order of the Knight.