In-depth articles exist for a wide range of topics (e.g., censorship, secret societies, e-waste, Legos, Taylor Swift) and call out longer-form content from what appear to be recognized and higher quality sources.
Google suggests that the new block will appear most often for broad topics and themes. In-depth articles will not appear for every query, however.
It’s not yet clear whether all publishers and sites will have equal access to this area via structured content markup or whether there’s some sort of internal white list of approved publishers.
It’s not yet live for me, but if Google presents the content as it did in the earlier test, the block will appear in the middle of the page.
Rather than in the middle of the page, I would argue this should be on the right rail with Knowledge Graph content.
Postscript: Google has confirmed the block will appear in the center of the page. The company also said there was no “white list” as I suggested above. Here’s a statement provided by a Google spokesperson:
“Our goal is to surface the best in-depth articles from the entire web. In general our algorithms are looking for the highest quality in-depth articles, and if that’s on a local newspaper website or a personal blog, we’d like to surface it.”