The island known as Site B in the Jurassic Park films is Isla Sorna.
In the franchise’s fictional history, Isla Sorna was used by InGen as a secondary facility for breeding and raising dinosaurs before they were transported elsewhere. This made it different from Isla Nublar, where the original Jurassic Park attraction was intended to operate.
Isla Sorna is the main setting of The Lost World: Jurassic Park and Jurassic Park III. The island’s abandoned laboratories, jungle terrain, and free-roaming dinosaurs provide the setting for both expeditions and rescue missions.
Fans sometimes mix up the two islands because both are part of the fictional Five Deaths archipelago. Isla Nublar is the park island most associated with the first film, while Isla Sorna is the production island commonly called Site B.