I saved my customized identifier into the KeyChain on iOS.It will be shared in devices that using the same appleid?

I saved my customized identifier into the KeyChain on iOS.It will be shared in devices that using the same appleid? I've tryed with three iPhones, they are diffrent device type.I use my appleid to Login in iCloud.First step, I save one customized identifer by objective-c codes.Then debug on the other iPhones,but i can't get the identifer that saved in the first iPhone. But my some users found that they use a new iOS device,their new devices can use the old customized identifier directly while they use thire appleid to active new device.I need to know why?