When is squawk 1000 assigned in USA?
Currently, FAA's "fusion" software uses discrete squawk codes to merge targets generated by different surveillance systems, i.e. SSR (often several of them) and ADS-B. They plan to upgrade the software so it can merge targets based on Mode S hex code as well, but (as of early 2019) that hasn't been rolled out yet. Once that is in place, ATC will switch to telling Mode S targets to squawk 1000. Mode A/C targets will still need discrete codes, though, including Mode S aircraft in areas that only have Mode C SSR.