The proposition consists of two parts. Part 1: Atik Yomin is clothed… This explains the concept of Atik Yomin being clothed in Arich Anpin. Part 2: In this respect… This explains in what respect Atzilut is said to be under Adam Kadmon.
Part 1: Atik Yomin is clothed in Arich Anpin in order to strengthen Arich in all its Sefirot… For obviously one who is clothed in and acts through another directs that other – the “garment” – to some goal suited to a unique capability possessed by the wearer of the garment. Thus Atik Yomin is clothed in all the Sefirot of Arich Anpin, and the purpose is to bind them all under the hidden root that, as we have explained, underlies the governmental order – the Unknown Head.