The following brief explaination on StallZ has been duplicated from the following tutorial from psyonix.com : Onslaught Mapping Tutorial
All credit for this goes to the author of this excellent tutorial. Onslaught Mappers please take a look at this tutorial.
Contents
Restricting Vehicles and Player Pawns
BlockingVolumes
The first and most important tool for keeping vehicles in the map are blocking volumes. Blocking volumes should be used to keep vehicles and players from accessing the boundaries of your map. Obvisouly you don't want to have a lot blocking volumes with nothing inside them so that they appear to be an invisible wall, but with Mantas and Raptors that can get over pretty much anything it is inevitable that at some point you have to just stop them short. See also BlockingVolume and Actor/Collision.
StallZ
The second tool is StallZ. This is a setting that you can find in the Level Properties that limits the height that a flying vehicle can fly. Once you reach this limit the vehicle is pushed back down to safer altitudes.
KillZ
The final tool is KillZ which serves as a backup just in case someone is able to get under the terrain or out of the map boundaries. Even if your map is very secure it is possible (although very rare) that a vehicle could be hit by a tank or something and somehow get smashed through the terrain. If you don't have a KillZ at that point you will have player walking around that is able to shoot people up through the ground.
Related Topics
Discussion
Tarquin: rename this page Mapping for Vehicles?
SuperApe: Nah, this looks like it should be a one-topic definition page, like those found through Terminology. For the topics of Mapping, there are a number of vehicle-relevant pages already in play. Should we have a separate page for KillZ? Rename this "StallZ And KillZ"? "Restricting Pawns"?