In truth, it’s the zivvugim of the Middle that are discussed in (the Ari’s analysis) of (the world of) Atzilut; the zivvugim of the Beginning and End are a whole other matter, though. They refer to the zivvug of Atzilut (itself) with the Kavod in (the lower world of) Briah.
For the (eventual all-encompassing) Tikkun will ultimately involve the attaching of the lower and higher (realms, worlds, beings, phenomena) to each other, after which everything will be attached to the Ein Sof, and it will be said that everything is one. That will entail the perfection of the Middle and the conjoining of the Beginning and End, and is the point of (our Divine) service.
For in fact the only things that can be differentiated from each other are the Lights and the (lower) entities; the Lights themselves can’t be differentiated.
The Ari often spoke of the conjoining of Partzufim. But as Ramchal wrote in a letter to his teacher, R’ Bashan, he himself had been allowed to speak of a greater conjoining of parts: that of the Beginning and End. For while the former address temporary, nearly temporal conjugations, the latter speaks to the ultimate conjugation of Creator and created. Our task here, his point is, is to bring about that very phenomenon; everything else is commentary.
(c) 2010 Rabbi Yaakov Feldman