every choice is a treatment the posted layouts already use, so anything you build stays in the family. generate applies your changes to the code on the right: colors, fonts, favicon and cursor edit the preset in place; a treatment change rebuilds the layout from the design model. "start from" loads a posted layout and matches the pane to it.
the box holds the exact text for the customize journal page: s1 style system, generator layout, overrides box under the options tab. edit it and the preview follows; "preview what is in the box" loads any code you paste in, copied from anywhere.