The regions, with their names and numbering are taken from
The Hewitts and Marilyns of Ireland by E D "Clem" Clements.
A Simm is a hill in England, Wales or Scotland over 600m high with a drop of at least 30 metres all round. Simm is an acronym for Six-hundred Metre Mountain. The Irish and Isle of Man Simms are also listed on this website.
A Subsimm is a hill which just fails (by up to 10m) to qualify on the drop rule, i.e. over 600m with 20-29m drop.
(Simms appear as Class=Sim on this website, and Subsimms as Class=sSim).