The document says "@[+]id/myid" as the definition for android:id, from the notation it indicates that the plus is optional. But nowhere can I find a definition of what the plus sign means or not supplying it would mean.
What does it mean and why is it there?
The documentation says,
The plus-symbol (+) means that this is a new resource name that must be created and added to our resources (in the R.java file). There are a number of other ID resources that are offered by the Android framework. When referencing an Android resource ID, you do not need the plus-symbol, but must add the android package namespace, like so:
android:id="@android:id/empty"
http://developer.android.com/intl/de/guide/topics/ui/declaring-layout.html
@+id/foo means you are creating an id named foo in the namespace of your application. You can refer to it using @id/foo. @android:id/foo means you are referring to an id defined in the android namespace. This namespace is the namespace of the framework. In this case, you need to use @android:id/list and @android:id/empty because these are the id the framework expects to find (the framework knows only about the ids in the android namespace.)
Taken from one of the "Android Developer | Google Groups" discussions.