By default, when the "exact" option is disabled Serpbook will return the highest ranking URL for the inputted domain. If the "exact" option is enabled, however, Serpbook will return the ranking only if the URL indexed in the search engine matches the URL inputted exactly.
This means that:
If the URL indexed by Google has a trailing "/" (for example, www.serpbook.com/serpbook/ ) but you're inputted URL doesn't have a trailing "/" (for example, www.serpbook.com/serpbook ), Serpbook will not register the ranking and it will return unranked ( "-" ).
If you are tracking a Facebook page with the "exact" option enabled, and the URL indexed by google is en-gb.facebook.com/serpbook/ but you entered www.facebook.com/serpbook/ on Serpbook, Serpbook will not recognize the URL and will mark it as unranked.
The exact feature is useful for tracking links like Youtube, or specific pages on your domain.
You can view the exact or broad, by going into Spyglass:
Then clicking on "View Keyword Settings"
And finally, you'll see "Exact ON?"