How to use level streaming and nav mesh with ai

he has a persistant level, he has a level called ai_navigation_geo, at the begining of the course he says the ai_navigation_geo level is for the navigation…but then he says “we want to make sure our persistant level is selected…if you build your nav mesh in a different level it will crash the game or it won’t work” also he doesn’t specify where he puts the AI characters…in the persistant level? so build my nav mesh on the persistant level and put all the ai in the persistant level?? it just doesn’t make any sence…i’ve gotten everything ready for my game but don’t know where to put the ai and nav mesh because of this course has confused me, it seems to me that i should be able to have ai and the nav mesh volume in level 1 and have ai and seperate nav mesh volume for level 2? but he says it has to be on the persistant level.