The accuracy of the proximity value, measured in meters from the beacon

应该是 iOS 系统根据一定公式计算出来的，是有考虑信号衰减曲线的

proximity 枚举值如下，应该仅供参考而已，因为安卓平台是没有的

/*

 * CLProximity

 *

 * Discussion:

 *    Represents the current proximity of an entity.

 *

 */

typedef NS_ENUM(NSInteger, CLProximity) {

CLProximityUnknown,

CLProximityImmediate,

CLProximityNear,

CLProximityFar

} API_AVAILABLE(ios(7.0)) API_UNAVAILABLE(watchos, tvos);