We all know the mission of al-Qaim

He will establish an Islamic state and apply Islamic laws that was revealed to the Prophet[1]

But you can also find some narration that states al-Qaim will also rule by the law of David and Jesus. What does this mean exactly?

Reference
[1] Nasr, Sayyed Hossein. "Expectation of the Millennium : Shiìsm in History,”, State University of New York Press, 1989, p. 19, ISBN 978-0-88-706843-0