大方廣佛華嚴經隨疏演義鈔 5.8

5.8

疏「開即為五」者，開三世間中智正覺一為三乘故，眾生世間及器世間仍舊不開，故為五也。

As for 'opening it into five' in the commentary: it is because within the three worlds, the one [world] of enlightened wisdom is opened into the three vehicles; the world of sentient beings and the world of the receptacle remain as before and are not opened, hence they make five.