Every SWF file runs at some unique level. Let us say we
have two swf files namely parentSWF and childSWF, with
childSWF loaded into parentSWF file.

As said, each swf runs at a unique level, assume that
parentSWF is running at _level0 and childSWF is running at
_level1. Therefore, _level0 will refer to the main
timeline of parentSWF and _level1 will refer to the main
timeline of childSWF.

_root will always refer to the maintimeline of its own swf.
In other words, if we say _root in childSWF, it will point
to the maintimeline of childSWF. Similarly, if we say _root
in parentSWF, it will refer to the maintimeline of
parentSWF.

It is therefore important to note that _root and _level0
will only be same if they are being referenced from the swf
file that is running at _level0. Similarly, _root and _leveln
will be same only in the swf that is running at _leveln.

As per the above explanation, if you refer to _root and
_level0 in an swf file that is running at leve5, the _root
will point to the maintimeline of this particular swf
(running at _level5) and _level0 will point to another swf
that is running at _level0.


Note: AS3.0 does not have any level concepts

Hope that helps :)