This posted in the Federal Register on 07/23/2019.
CBP faces significant challenges in targeting Section 321 shipments, while still maintaining the clearance speeds the private sector has come to expect. This is because CBP does not receive adequate advance information in order to effectively and efficiently assess the security risk of the approximately 1.8 million Section 321 shipments that arrive each day.
More info on the Pilot can be found here.
Section 321 - E-commerce Pilot - (Data Pilot)