The $$SEARCH$$ directive inserts a search-for dialog comprising a text input field and a submit button immediately adjacent to it.
The (on-screen) size of the text field can be specified as an optional dollar-delimited parameter, then an alternate label for the submit button as another option. In this way the overall width of the search dialog can be adjusted according to the space available at the location.
As described in Icons and illustrated by the Vdub (this) documentation the search submit button can be replaced by an icon (image).
This reserved section is used to report search results. It can only be seen in the table of content when in site management mode.