There are three reserved sections and titles. There is one other similar used during site editing. These do not appear in the table of content or site-map unless in management mode.
This section provides the content required to login. When login is requested this section is substituted for the $$CONTENT$$ of the current section. Although additional site specific information or instruction can be provided the essential HTML form requesting and submitting the login credentials must be preserved.
When a keyword search has been submitted this section is used to report the result.
When a section is requested and not found this section is used to report the issue.
This section exists after a site edit copies or removes a section. It can then be re-inserted at another point in the document or subsequently completely removed (deleted) from the edit. Only one such clip can exist at a time; a subsequent copy or remove overwriting any existing clip.
Renaming an existing $$CLIP$section-title$$ to include an additional identifier, perhaps an obvious number as in $$CLIP-1$section-title$$, is one method for maintaining multiple concurrent clipped sections. Renaming back to $$CLIP$section-title$$ allows that clip to then be reinserted or deleted.
Any section name beginning with a double-dollar does not appear in the table-of-content unless managing the site. For example, the section $$CHANGES$$ could be used to maintain a log of site changes obvious only to the site maintainer.