When you set up the Messages app in iOS or OS X, you must supply it with an active Apple ID. But a single Apple ID can be associated with multiple phone numbers and email addresses, and the iMessage system enables you to send and receive messages to and from any email address or iPhone number that’s associated with that ID.
That can have a lot of repercussions whether you’re setting up the Messages app in iOS (on an iPhone or other device) or in Mac OS X 10.8 Mountain Lion. For example, on an iPhone, you could opt to receive only those iMessages that are directed at its phone number. At the same time, you could configure the Messages app on a Mac to