Actions
Doom 64 has various actions which cause levels to transform, doors to operate, lights to change, and other events. These actions can be assigned to linedefs and activated in many ways such as crossing, shooting, the use button, or another type of trigger. Monsters will be able to activate most linedefs they cross if those linedefs have the "Trigger By Dead Thing" flag (this does not work on the original Doom 64 EX). If you are using linedefs to trigger actions, they will appear yellow on the player's automap. To hide this, you can check the "Hide Special on Automap" box on the lindef properties and they will appear as a normal linedef. This can also be a good way to hide secret doors too. Below is a complete list of actions available in Doom 64.
Whichever linedef is tagged 999 will be activated upon map startup as if triggered by the player. This is how the intro camera sequence for Doom 64 is activated right from the start which happens on map33.
Scripted Sequences
Also Doom 64 introduces scripted sequences in the form of macros and monster tunnels. Macros are used to execute multiple actions with just a single linedef activation. The actions can happen all at once or delays can be added between them to create a sequence. Only one macro can be run at a time within the engine - all other macros will be blocked until the current macro is finished. On the other hand, monster tunnels can also sequence actions too and multiple monster tunnels can be active at once. But monster tunnels cannot replace macros completely. The links below demonstrate macros and monster tunnels in more depth.