I've often wondered that, but just assumed it wouldn't make sense in english, only german.
I've also assumed that the P is for "plus" or "improved" or "premium" or something like that, since the ET200SP is replacing ET200S and the ET200MP is replacing ET200M.
I dunno if the brochure is accurate, or if thats something made up by the guy who wrote the brochure. It doesn't make sense to me to go from old gen hardware as "ET200 Scalable" to new gen as "ET200 Scalable Periphery".