The U.S. Department of Transportation has made a major revision to CFR 49 Section 173.309 Fire Extinguishers. The revision was made to the opening section which describes what the US DOT classifies as fire extinguishers. This DOT description is not based on product use, NFPA, or other fire-related definitions. The DOT regulates the manufacturing, testing, and transportation of what it describes as fire extinguishers.
This revision makes pre-engineered and engineered fire extinguishing system cylinders "fire extinguishers" for transportation purposes. With this being the case, pre-engineered and engineered system cylinders would be required to be marked, labeled, and entered on the shipping papers as Fire Extinguisher, UN1044, Hazard Class 2. NOTE: this does not include any carbon dioxide cylinder, extinguisher, or cartridge. Carbon dioxide must transported, labeled, and marked as Carbon Dioxide, UN1013, Hazard Class 2.
Additionally, fire extinguishers and systems cylinders that are cartridge or cylinder operated may be transported as a fire extinguisher with the following limitations: the cartridge/cylinder must be part of the full assembly and as referenced in the DOT Interpretation 19-0052, the cartridge/cylinder must not contain carbon dioxide.
For additional information on packaging, please refer to DOT Interpretation 12-0075 which describes what the DOT considers as acceptable packaging of non-specification (cylinder) fire extinguishers.
Below is the text of the updated CFR 49 Section 173.309. The underlined areas are the revisions. For reference, you can access the full text here.
CFR 49 §173.309 Fire extinguishers.
This section applies to portable fire extinguishers for manual handling and operation, fire extinguishers for installation in aircraft, fire extinguishers for installation as part of a fire suppression system, and large fire extinguishers. Fire extinguishers for installation as part of a fire suppression system include cylinders charged with either a compressed gas and an extinguishing agent or a gas which comprises the sole fire extinguishing agent in the system. A fire extinguisher does not include cylinders pressurized with a gas for purposes of expelling a separately stored extinguishing agent in the fire suppression system. Large fire extinguishers include fire extinguishers mounted on wheels for manual handling; fire extinguishing equipment or machinery mounted on wheels or wheeled platforms or units transported similar to (small) trailers; and fire extinguishers composed of a non-rollable pressure drum and equipment, and handled, for example, by fork lift or crane when loaded or unloaded. Cylinders filled with a compressed gas whose purpose is to expel a separately stored extinguishing agent may not be transported under this section when offered for transportation or transported apart from a suppression system.