A Border Crossing is a building that allows you to control the movement of people and goods across the border. If a county has a border crossing, any person wishing to go through the county abroad must wait until one of the citizens controls him and allows him further.
Gondor builds a border crossing at Emyn Arnen. That means that anyone wanting to go from Morgul- held Barad Othnir to Emyn Arnen or other way round, will have to write themselves at the bottom of the Emyn Arnen page under the ==Border Crossing== heading and wit until a Gondorian arrives. Depending on Gondorian laws, the passport check might demand customs (money), if not then they will take resources.
(if customs leaving are 5% on Silver, and the person is taking 100 silver to Barad Othnir, the border control demands 5 Silver pieces, from the tourist, or it can be paid in currency units.
Building BCs is nonsense if you do not build them around the whole border length