For car journeys we offer multiple ways of choosing the type of vehicle. In certain countries people are more familiar with fuel economy figures - MPG or L/100km - for those countries car journeys can be calculated that way. Elsewhere, "by size" and "by segment" are offered as categories.
By size relates to the size of the car's engine and the categories are as follows.
< 1.4 litres
1.4 - 2.0 litres
> 2.0 litres
< 1.7 litres
1.7 - 2.0 litres
> 2.0 litres
By segment gives more options and Defra gives examples for typical makes/models for each segment as follows.
Bentley Continental GT
Dual Purpose 4X4
Land Rover Discovery
For motorbikes the engine sizes are:
125cc to 500cc
According to Defra "The conversion factors [for cars] are based on information from the DfT (Department for Transport) who regularly analyse the mix of cars on the road in Britain through DVLA records and automatic number plate recognition (ANPR) data. The conversion factors are updated each year to reflect changes in the spectrum of cars of different types and ages being driven."
The emissions factors for cars assume an average occupancy of 1.6 people.
You can download Defra's methodology paper which gives details of how all of their factors are derived.