What's the meaning of @+android:id in layout file.

[biAji]

hi guys,

    I recently found this attr in some app's layout files, such as Phone & Settings. I fully understand @+id or @android:id. But what on earth of @+android:id means? Add id to android namespace?  Is it possible?

    And more:  How does aapt tool deal with this attribute value?  And where these progress happen? 

     Thanks!