PRI stands for Prime. These petcocks had a vacuum shutoff that cuts off the flow of gas when the engine is not running na d in the ON or RES (reserve) position. In the PRI position thought the petcocks allow gas to flow to the carbs to get the gas up to the right level to get started after the carbs had sat a while and the bowls dried out.
BTW, these petcocks are notorious for leaking. Search the web for solutions