Value of "nonresearch articles" section in PNAS
I had a look at the latest collection paper (Megafauna and ecosystem function from the Pleistocene to the Anthropocene). This turned out to be a PNAS Direct Submission, which appears to be a pretty standard peer-review process and definitely the standard way to submit papers to PNAS ("Direct Submissions now account for more than 95% of papers submitted and 78% of papers published in PNAS"). I couldn't find out any sort of invitation for collected papers, so my guess is that these are papers that have already been published through standard peer-review, which have been grouped after the fact by PNAS editors. So this isn't a "PNAS section", but just a thematic collection of papers that have been normally published in PNAS (which, in 2014, had an acceptance rate of 17%).