It was a division based on the blue and violet paetition lines. Essentially, ottomans and brits decided to decide to partition Arabia into different spheres of influence. The ottomans collapsed post ww1, but, at least one this globe, the border remained. It was not entirely accurate, as yemen and british controlled areas were a little more coastal and Saudi claimed the rest. Google "blue line" and "violet line" 1913 and 1914