This web part is an enhancement of Kentico Editable Text web part. It allows you to display the same content on different pages and different websites.
The external source could be an another EnhancedEditableText or EditableText WebPart with the same ID.
It can be used if certain text needs to be repeated in different pages or different sites. This text can be then edited on one place.
Web Part Properties settings:
If the ExternalSource check box is checked, the content will come from the website selected in the SourceSiteName field, from the page selected in SourcePath field and from the EditableText web part with the same ID and culture as the actual web part. In this case, the content of the actual web part is shown as read-only on Edit tab and cannot be edited.
If the web part setting above is pointing to itself, the content can be edited and will be stored in the same page even though the ExternalSource check box is checked. This allows to use the same page template in more pages and have just one text source.
If ExternalSource is "off", the content can be changed and it will be stored in current page - this WebPart behaves like a regular EditableText WebPart.
If ExternalSource is "on", the content form external page is stored in current page document as well but it is not editable. This is to allow the search engine to index it correctly.