The center attribute MUST return the value it was initialized to. When the object is created, this attribute MUST be set to a GeolocationPoint instance with its latitude and longitude properties set to the same values as those properties in the CircularGeofenceRegionInit dictionary. This represents the center of the circular region. Latitude must be between -90 and 90 inclusive. Longitude must be between -180 and 180 inclusive. If either of these properties is outside these ranges, the constructor will throw a RangeError.
The last sentence is describing algorithmic behavior, which should be part of some steps/algorithm and not defined here.
Spec says:
The last sentence is describing algorithmic behavior, which should be part of some steps/algorithm and not defined here.