The atmosphere type, along with the AtmospherePressure, define the nature of the world's atmosphere. Atmosphere classifications are deliberately broad, mostly because working out anything more detailed is far more complex, and would probably lead to less realistic results because of errors.
Inert - Inert gases and vacuum worlds
Breathable - Earth-like atmospheres which can be breathable.
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Cold - Atmospheres of cold terrestrial worlds.
Deadly - Deadly atmospheres which need special suits.
Exotic - Anything which doesn't fall into the above.
Many atmospheres require some level of protective clothing, which may range from a simple breathing mask, right up to a mini-tank. The general level of protection required can be described using the standard Imperium Ratings Code, as detailed below:
I: A simple filtering mask. Does not provide its own oxygen supply.
II: An oxygen mask, rated for VeryThin or thicker atmospheres with little or no oxygen.
II/b: As II, but includes eye protection.
III: Full skin protection, suitable for VeryThin or thicker atmospheres.
IV: Vacuum suits, always contain their own oxygen supply and provide some protection against radiation and poisonous gases.
V: Hard suits, for protection in highly radioactive or corrosive environments. Bulkier than standard vacuum suit, but not greatly so.
VI: High pressure suits, very bulky, very difficult to move in. Anything more than this is considered a vehicle in itself.