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.
Class 4
Flammable Solids or Substances
Read the divisions below as a planning note. The class name tells you the main hazard. Packing, labels and where the box may sit on the ship still come from the IMDG Code.
Flammable solids, class 4
Division 4.1. Flammable solids
A flammable solid catches fire easily. Powder, granules or a paste can light from a short touch with a flame, such as a match, and the flame then spreads quickly. Friction can start the fire too.
This division also covers a self-reactive substance, which can break down and give off a lot of heat even with no air; a solid explosive that has been wetted or diluted so it no longer behaves as an explosive; and a substance that, without a stabiliser, can polymerise and heat up under normal transport conditions.
Division 4.2. Spontaneously combustible solids
A spontaneously combustible material is pyrophoric: a liquid or solid that can ignite within five minutes of meeting air. It also covers a self-heating material that, in air and with no outside energy, can heat itself.
Division 4.3. Dangerous when wet
A material that, on contact with water, can catch fire by itself, or can give off flammable or toxic gas faster than 1 litre per kilogram of material per hour.
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.