Z-clipping

Z-clipping was a feature present in Doom, but only partially. It was fully implemented in Hexen. It consists on using the height and z-coordinates of a thing for allowing any other actor to pass above or below and, for a falling actor, to receive falling damage depending on the height and speed of the fall.

Doom and Heretic featured a limited z-clipping that was only true for projectiles and for allowing certain things, like monsters, to be able to enter a sector depending on it's ceiling height. Aside of that, a given thing's height or z-coordinates were ignored by the engine, causing all actors to be infinitely tall. As a result, a player may stand on a ledge of 640 floor height, and a blocking decoration or monster standing just in front, on a floor of height 0 will completely block the player from jumping, and in the case of monsters, these are able to perform their melee attack on the player succesfully.

In Hexen, however, this is different, as a player can normally stand above monsters, but z-clipping was mostly implemented for allowing players to stand above floating bridges. These bridges are things spawned via ACS that under some circumstances will allow the player to reach an area that was not accessible before.