BodyId and BodyClass

This pair of setting allows to specify an Id and/or Class name that will be added to the body of the edited document (not in full page mode). Additionally that same Id and/or Class name will be applied to the Style and Format dropdown combos.

Example: your body background-color is black, but the content that is edited is placed inside a <div id="news"> element so you have this CSS

body {
background-color:#000;
}
h1 {
color:#FFF;
}
#news {
background-color:#FFF;
}
#news h1 {
color:#333;
}

by specifying in your config file:

FCKConfig.BodyId = 'news' ;

the editing area will have a white background and the heading 1 elements will be in dark color, so you can reuse the same css file for your site and the editor.