Cross Section Drawing Seed - OpenRoads Connect Edition

Hello, 

I have watched the video below which goes over how to create a cross section sheet seed.

 https://learn.bentley.com/app/VideoPlayer/LinkToIndividualCourse?LearningPathID=113449&CourseId=122400&MediaID=5014676

Can someone further explain what the Top and Bottom clearance works?

The video mentions it is for annotation but how do you know what quantity to pick for each project? 

Are the numbers feet ( Imperial)? Is it some sort of scale? 

Thank you!