Classification is a property of the substance
Whether goods are dangerous for transport purposes is determined by their physical and chemical properties against a defined system of classes and criteria. It is not a commercial judgement, and it does not depend on how much is being sent or how carefully it is packed.
Some substances are obviously dangerous. Many are not: aerosols, batteries, paints, adhesives, some cleaning products, some machinery containing fuel or a battery. The items that cause problems are usually the ones nobody thought to ask about.
The obligation to establish it sits with the consignor
The party offering the goods for carriage is responsible for determining whether they are dangerous, classifying them correctly, and declaring them. A carrier is entitled to rely on that declaration and is not in a position to test it — nobody at a loading bay can determine the flashpoint of a liquid in a sealed drum.
That allocation is why undeclared dangerous goods are treated seriously. They are not a paperwork omission; they put a load on a vehicle that is not equipped for it, with a driver who has not been trained for it and emergency services who would not know what they were approaching.
The regime covers far more than a document
Road carriage of dangerous goods in Europe is governed by the ADR agreement, which addresses which substances may be carried and in what quantities, how they must be packaged, how packages must be marked and labelled, how the vehicle must be placarded and equipped, and what training the driver must hold.
The transport document records compliance with all of that. It does not create it, and a correctly completed document accompanying incorrectly packaged goods is not a compliant movement.
Limited quantities and exemptions exist and are not a general escape
The regime contains provisions under which smaller quantities packaged in particular ways attract reduced requirements. They are precise, they are specific to the substance and the packaging, and they are not a category a shipper can place themselves in by judgement.
Where a consignment might fall under such a provision, that is a question for a qualified dangerous goods safety adviser rather than an assumption to make. Getting it wrong produces an undeclared consignment with a rationale attached.
Each mode has its own regime
ADR governs road. Rail, sea and air each have their own set of rules, with their own classification detail, their own limits and their own documents. A consignment moving by more than one mode may need to satisfy more than one regime, and satisfying one does not satisfy another.
This matters for anything that starts as a road movement and continues by sea or air, where the requirements at the second leg can be stricter than at the first.
The limits of this explanation
This is an explanation of where the obligations sit, written so that a shipper knows which questions they own. It is not a classification guide, it contains no thresholds, and it is not a substitute for advice.
The rules are detailed, they differ between the four transport modes, and they are revised on a regular cycle. A shipper who is unsure whether their goods are in scope should put that question to a qualified dangerous goods safety adviser before the consignment is offered for carriage, not after it has been refused.