Specifically referring to the "Application to Transfer and Register NFA Firearm".
A specialized version of the standard MP4 file format used for efficient internet streaming and faster processing of video data. Deep Dive into Fragmented MP4 (fMP4) EFAFPmp4
Could refer to a specific step in the "train stop" navigation of the eForm 4 system or the requirement for a digital photograph. Specifically referring to the "Application to Transfer and
If your inquiry is focused on the technical file format itself, represents a significant shift from traditional MP4 files: Video Tutorial - eForm 4 - ATF EFAFPmp4
Commonly used by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) for their online submission system.