Unity uses A* to calculate the shortest path on the NavMesh. Nodes and links visited during pathfinding. To fully understand how the cost works, let’s take a look at how the pathfinder works. For example, if you set the cost of an area to 3.0, traveling across that area is considered to be three times longer than alternative routes. In a nutshell, the cost allows you to control which areas the pathfinder favors when finding a path. The area type can be assigned to every object that is included in the NavMesh baking, in addition, each Off-Mesh Link has a property to specify the area type. Door area is made accessible by specific characters, to create a scenario where humans can walk through doors, but zombies cannot.Water area is made more costly to walk by assigning it a higher cost, to deal with a scenario where walking on shallow water is slower.In the above example the area types are used for two common use cases:

More info See in Glossary Agent has an Area Mask which can be used to specify on which areas the agent can move. In addition each NavMesh A mesh that Unity generates to approximate the walkable areas and obstacles in your environment for path finding and AI-controlled navigation. The Navigation Areas define how difficult it is to walk across a specific area, the lower cost areas will be preferred during path finding.

Unwalkable override ue4