The Separator macro, is different from other macros.  It does not need to be enabled in your WebGUI config file.  It also can only be used with two Assets.

In the Article, the Separator macro can be used to build a multi-page Article, where the macro is used to indicate page boundaries.  Simply put the macro where you want one page to end, and another one to begin.

In a Post or Thread Asset, the Separator macro can be used to select the Summary for the Asset.  Everything from the beginning of the Asset content to the Separator macro will be taken as the Summary.

Recently (July 2009), the Separator macro was updated so that you can safely put it in the middle of HTML tags.  The macro will take care of creating ending and starting tags, as required to make good, valid HTML content for each page or section of content.