The super parent for all the movieClip is _root
but using the _parent you can get particular MovieClip's parent.

For example you are having one Movie Called MovieHolder_mc
and MovieHolder2_mc on Stage, and inner1_mc inside the
MovieHolder_mc. Now you can access the MovieHolder in Two Ways


_root.MovieHolder2_mc (Direct access)

to get this from the innerMovie 

inner1_mc._parent._parent.MovieHolder2_mc

_root is used to access outer Movieclip.

_parent is used to access Moviclip inside having the 
moviclip.

_parent is used to access the parent display
objects/properties of a MovieClip/Button. its something like
the relative path to the current MovieClip/Button. _root is
like the absolute path irrespective of the location of the
current MovieClip/Button.

_root to specify a absolute path.

_parent to specify a relative path to MovieClips.

_root - root timeline of the current swf.

_parent - the object containing the current object.

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_root is the head tag for all inside the parent tag and
child tags.but _parent is the particular limitedtag only.

_root is refernce root timeline of movieclip


_parent is refernce current movieclip or object

_root = Main Gate Key
_parent= Inner Room Gate Key

_root is the direct access.
_parent is reference of one outer movieclip