IMDG
Dangerous goodsInternational Maritime Dangerous Goods Code
The IMO rulebook for shipping dangerous goods by sea.
In practice
Covers classification, packaging, marking, labelling, documentation, stowage, and segregation on the vessel. Updated every two years. Ocean DG declarations must follow it, and the carrier's DG desk checks every booking against it.
Seen in the wild
Stowage category as per IMDG: on deck only, away from living quarters.