What does object annotation mean?

Possible duplicate:
What is the difference between scala subclasses and type types?

I understand self annotation as a promise to the compiler, where the programmer shows that the trait will mix with the annotated one. For instance:

scala> trait X
defined trait X

scala> trait Y { this: X => }
defined trait Y

scala> new Y {}
<console>:10: error: illegal inheritance;
self-type Y does not conform to Y selftype Y with X
              new Y {}
                  ^

scala> new Y with X {}
res1: Y with X = $anon$1@1125a40

In the previous example, the third expression failed because we did not set the actual X to a new instance. Obviously, the latter works well. So far, so good. And now let's look at another example that includes an object.

scala> object Z { this: X => }
defined module Z

I understand that the object is created with an error with the promise of X (we are creating an instance now with a future promise!), As shown in the following lines, where the properties have been slightly changed:

scala> trait X { class X1 }
defined trait X

scala> trait Y { this: X => new X1 }
defined trait Y

scala> object Z { this: X => new X1 }
<console>:8: error: not found: type X1
       object Z { this: X => new X1 }
                                 ^

So what does object annotation mean?