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Protected region can refer to all possible orbits around a celestial body that are stable. For example, it may be that two orbiting planets' orbits are sufficiently separated that a planet could orbit between them. An orbit too close to one of the two would be affected by that planet and the orbit would not last. A region between two planet orbits that is sufficiently far from both orbits for a lasting orbit of yet another planet is a protected region.
The term protected region is also commonly used in an unrelated sense regarding man-made satellites: they are desirable regions of possible Earth orbits where it has been agreed that satellites should removed after their useful life, to allow room for future satellites, minimizing the risk of collisions.