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(Version 5) How Classification and Labelling types represent GHS history
Classification and labelling types follow two different approaches in order to teach readers/implementers about the changes in GHS's history.
Hazard classes and categories are initially represented in enumeration types per class, with its name including a reference to the GHS revision it introduced. Whenever a class is modified in a new GHS revision, an independent new enumeration type is defined. The most complex case is the Flammable Gas class, which was part of the original GHS (type GhsRev0ClassFlamGas), then enhanced with additional classifications for ûnstable gases in GHS revision 4 (type GhsRev4ClassChemUnstGas) and pyrophoric gases in revision 6 (type GhsRev6ClassPyrGas).
These additional classifications could only be applied if the product is classified as a Flammable Gas, so in the printed GHS, they belong to this class. However the idea of class-specific enum types is that you can choose just one per class, so the additional classifications were implemented separately. GHS revision 7, well, revised this very untypical concept. All three were replaced by a new definition of Flammable Gases which integrated unstable and pyrophoric gases nicely (type GhsRev7ClassPyrGas).
When you have the class-specific enumerations defined, you still need a type to represent the full list of classifications of a product. SDScom definies some Ghs0XClassCategoryEnum for this, with X being the number of the GHS revision. Technically, this is a union of the class-specific types mentioned above. Although now the schema cannot enforce this, as an implementer you still now that the user should be able to pick just one of each class-specific type.
So you can easily tell in which original GHS revision a specific class was introduced or modified. E.g. for GHS revision 7, Explosives were last modified in GHS revision 1, Aerosols in revision 4, Oxidizing Gases are still unmodified, and revision 7 changed Flammable Gases as already mentioned. If you want to know what changes when a country mnoves from GHS revision X to Y, this is a good place to start.