The 6x6 posts are set by your installer, according to the specifications called out in your Prowell dimensioned drawings. The post placement is determined by the gate, as well as the side Foyer Panels. Once the posts are cut to length, the Arbor Assembly #20 is set onto the tops of the posts as a single assembly. The bottoms of the arbor posts are either equipped with mounting caps, or as in this example, with 5/8" threaded rod bored into the bottom of the arbor posts and a corresponding bore into the top of the site posts created by the site installer.

Arbor #20 is everything above the joint line on the posts.