OpenCalais is an external Suggestion Provider, used in Enterprise for scanning the content of a Publish Form for articles aimed at publishing to the Web. After scanning the content, suggested tags (keywords) are generated which are subsequently displayed in the Publish Form where the user can optionally include them in the article.
Figure: A field in a Publish Form for adding tags (A) and the tags suggested by OpenCalais that are displayed next to it (B).
The OpenCalais functionality is provided through a Server plug-in named "OpenCalais Suggestion Provider".
For information about configuring OpenCalais in Enterprise, see Setting up the auto-suggestion feature for adding tags.