Hi!
Special Sentences are list of terms managed by the user that (usually) shall be avoided in the requirements because they are ambiguous, indefinites, imprecise, open ended, etc…
Once you have created your Special Sentences list and added some terms, it will automatically create a Correctness metric in the metrics baseline. Then you decided how is the proper quality function for the metric (High, Medium, Low) and you can start using them.
Hope you find this information useful