FCKeditor is the default editor in CMSimple_XH before version 1.5 (if you're using a newer version, read the chapter about tinyMCE). It recognizes automatically the setting of CMSimple concerning (X)HTML and produces accordingly HTML or XHTML compliant code.
Moreover FCKeditor4CMSimple possesses an internal link funktion, which makes linking to internal pages easy.
It is recommended to keep FCKeditor as online editor.
A general extensive user guide for FCKeditor is available. But be aware that this guide doesn't cater for particular CMSimple_XH features, such as the special meanings of the heading format and the adapted link and image browsers.