The page Site.Preferences contains customisable browser preference settings. These include access keys (keyboard shortcuts to certain actions like edit, history, browse) and settings of the Site.EditForm (width and height of the edit textarea) as well as the name of the edit form in use.
A different page than Site.Preferences can be chosen by making a copy of that page under a new name, customising it, and setting a cookie which will point to this page for the browser being used, through
?setprefs=SomeGroup.CustomPreferences
SomeGroup.CustomPreferences being the name of the new customised preference page.
Note that in order to enable parsing of Site.Preferences by default, a line like the following needs to be added to local/config.php:
XL Page?('prefs', "Site.Preferences");