Here's something interesting I found in Flash CS3 using ActionScript 3. Say you have the following:
A MovieClip instance named "my_mc"
The output is:
So Flash assigns the property identifier "my_mc" to the current timeline's instance at the start of the movie, before the actual instance occurs on the stage. Then when it does, that property is populated with a reference to the instance. I assume the identifier is created at compile time.