You can find the official PDF through GovInfo or the Federal Register archive. Option 2: IATA Lithium Battery Guidance
This is the most likely match for a specific document download search. is a rule published by the Federal Aviation Administration (FAA) . Download 21681 pdf
The official guide is available at IATA.org . Option 3: Other Technical Standards If neither of the above fits, "21681" might refer to: You can find the official PDF through GovInfo