It is the IMO rulebook for dangerous goods on a ship: how to class them, pack them, mark them and stow them. Each substance gets a class, a UN number and packing rules that the carrier and the terminal have to follow.
Sea cargo reference
Nine hazard classes for cargo that goes by sea
Every dangerous substance booked on a ship sits in one of nine classes. The class names the main risk. It does not replace the packing, marking and stowage rules in the IMDG Code.
Explosives, class 1
Division 1.1. Mass explosion
These explosives can set off almost the whole load at the same moment. That blast is called a mass explosion.
Division 1.2. Projection, no mass explosion
These explosives can throw fragments. They do not blow up nearly the whole load at once.
Division 1.3. Fire hazard, no mass explosion
The main hazard is fire. There may also be a small blast, a small projection of fragments, or both. These articles do not set off almost the whole load at once.
The fire can throw off a lot of radiant heat, or the articles can burn one after another and produce that small blast or projection.
Division 1.4. Minor blast, mostly inside the package
The effect stays largely inside the package. Pieces of any real size are not expected to travel far. A fire outside the package must not set off almost the whole contents at once.
Division 1.5. Very insensitive explosives
These substances can still produce a mass explosion, but they are so hard to start that, in normal transport, they are very unlikely to ignite or to turn from burning into a detonation.
Division 1.6. Extremely insensitive articles
Articles only, with no mass-explosion hazard. They contain extremely insensitive detonating material, and the chance that they start by accident, or that a blast spreads from one to the next, is negligible.
Questions
What a class does not settle
The class names the main hazard. Many goods also have a second one, such as a poisonous gas that can also burn. The papers and the placards show both, and the separation rules follow whichever hazard is stricter.
It belongs to the substance, or to a small group of similar substances, in the IMDG Dangerous Goods List. It is not one number for a whole class. Two boxes in the same class can have different numbers, packing groups and separation rules.