From CSMS #17-000056:
Effective immediately, importers filing ATF information electronically through ACE are required to use the PGA Message Set. DIS images of ATF Forms, including Form 6 and 6A, will no longer be accepted. When the PGA message set is filed correctly, the importer will received an automatic “May Proceed” from ATF, and if there are no further issues, the shipment will be released. Please note, original paper ATF Forms 6 and 6A may also be submitted to the ports of entry. For further information about PGA forms and filing procedures, visit CBP’s webpage on this topic.