CL65 First Officer reports confusion over the MDT airport diagram depiction of the displaced threshold. Crew departed from Taxiway B thinking it was the end of the runway.

Narrative

We taxied to Runway 13. We had consulted the airport diagram and it appeared that the end of the runway was unusable. We were cleared for takeoff. After we took the runway ATC then informed us that we were not at the end of the runway even though we were at the runway threshold. They provided us with distance remaining from that point. We were under the impression that the performance numbers were good from the runway threshold and therefore continued with the takeoff. No further action required. Takeoff occurred without incident. If you look at the airport diagram for MDT Runway 13 you will see the markings on the extended portion of the runway between Taxiway A and B. These markings make it look like that portion is unusable. If you look in the Introduction section of the Jeppesen there is no description of what these markings mean. There needs to be clarification of these markings or a note at that end of the airport diagram explaining the usefulness of that portion of the runway.