I'm not sure what this means exactly, but I suspect that it may relate to SBRs?
Objective Factors for Classifying Weapons With “Stabilizing Braces”; Withdrawal of Guidance
The Bureau of Alcohol, Tobacco, Firearms, and Explosives (``ATF'') is announcing the withdrawal of a notice and request for comments entitled ``Objective Factors for Classifying Weapons with `Stabilizing Braces','' that was published on December 18, 2020.
www.federalregister.gov