The numbers inside of the grid pattern indicate the ‘predetermined’ safe altitude to fly in that particular area of airspace. This means that to receive the quickest response to your request for an airspace authorization, you need to fly at an altitude below the ‘indicated’ altitude in the area where you’re wanting to fly.

You’ll find the larger numbers appear on the outer areas of the grid, and as you get closer and closer to the airport, you’ll see numbers like 100, 50, and even 0. If you see a ‘0,’ that doesn’t necessarily mean that you cannot fly in that area, but it will require extra time to coordinate your request with the airport’s ATC before receiving approval to fly.