A Hewitt is "a Hill in England, Wales or Ireland over Two Thousand feet high (610m) with a drop of at least 30 metres (98 feet) all round".
The Hewitts of England and Wales are therefore a subset of the Nuttalls.
A Subhewitt is a hill which just fails (by up to 10m) to qualify on the drop rule, i.e. over 610m with 20-29m drop. Note that the subHewitts formerly included hills between 600m and 610m, but these are now simply classified as Deweys.
(Hewitts appear as Class=Hew on this website, and Subhewitts as Class=sHew).
The list was compiled and is maintained by TACit Press, who also publish a hillwalking fanzine, and other interesting stuff about mountains.