Exemption
T, S, U and D exemptions
The short answer
A waste activity small or low-risk enough to run without a full permit. It still has to be registered with the regulator and is capped by tonnage and by what it covers.
In practice
Exemptions are narrow and are the thing most often outgrown quietly. A site that has grown past its tonnage cap is operating without a permit, not operating under a generous one.