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