On the map, you are able to see different layers of the prison. Each floor is labeled according to its vertical level (Floor 1, Floor 2, etc.) The highest floor of a building is called a roof. Vents and the underground are two more types of layers on a map - underground is the bottom layer, while vents show up between floors.
Floors can be traversed in a variety of ways. The legal way to go up and down floors is to climb the stairs (see image on the right). The green arrow on the top symbolizes the direction the stairs lead to (in this case it is up). Alternatively, vents can lead you to rooms on a different story. Grappling hooks are used to hitch you up to the roofs of buildings, and your physical body can withstand a drop down 1 floor.
Floors are generally unable to be interacted with, since you cannot modify them like walls or fences (unless you are on floor 1 and you can dig underground). However, they will have vents that you can use to transport yourself to forbidden areas. Indoor buildings can shield you from sniper guards outside.
For the mechanic to dispose items, see Dropping.
To drop down a floor, just use the "move" controls to walk off the edge. Make sure you are dropping somewhere without any hindering objects, such as a fence or else it will fail and your character will say "I can't descend there!". To descend a drop of more than one floor, you need a Length of Rope or Sheet Rope. If you dig a Tunnel, standing on top of/underneath the entrance hole will show a prompt that allows you to interact with the hole to go up or down.
To scroll through different floor, roof, vent, and underground map levels, press [Page Up and Page Down]