Critical National Infrastructure (CNI), or sometimes just referred to as Critical Infrastructure, is any service which assists in the function of communities.
CNI typically includes the following industrial sectors:
Energy
Electricity, including nuclear and renewables
Oil, Gas, Coal
Water
Drinking water
Waste water
Transport
Road
Rail
Air
Water
Other sectors are sometimes included in CNI, including:
Healthcare
Financial sector
Telecommunications
Emergency services
Agriculture
Critical manufacturing, such as pharmaceuticals and chemicals
As such a critical service is critical to society by very definition, they are a target for APT groups.