MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
O is 1-48 or 1 inch is 48 inches on the model
HO is one half O or 1-87,
N is 1-160
or take a look at this handy page
I hope this helps you