Bkcover commands are used /Uonly/U in the 'bkcover' file to define:
/FLSections Tabs/FB - these appear along the bottom of the notepad. Their colors are defined using the //CT command and the tabs are defined using the jump command, //JB.
/FLCover/FB - you can assign a color (//CC command) or a picture (//IC commnad) to the cover of a notebook (select Contents and then Page Up!!).
See the bkcover file in this example to see how the commands are used!