Sea to Summit says:
The Thermolite Reactor increases sleeping bag performance by up to 8°C with 80g/m² Thermolitefabric. It is ideal as part of a cold climate sleep system, or as a standalone bag in warm weather. The Thermolite Reactor is made from the lightest of Thermolite fibres.
The liner is made from Du Pont Thermalite fabric this is a hollow core fibre that provides maximum insulation for weight. If you want a liner to keep your bag clean then this must be a consideration as it offers an insulation upgrade at the same time.
Could be used on its own if camping in a warm climate.