Question

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?

Answer 1

@+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.