A proprietary game engine developed by Pandemic Studios, first used Battlezone II: Combat Commander and later used in several Star Wars games including the popular Battlefront series.
The main factor which limits modding abilities with Zero Engine is that MemoryPools are stored in the engine code itself, and not changeable since access to the source code remains closed to this day. Some of these limitations are described here Info.swbfspy.org
More about ZeroEngine:
Psych0fred - "The engine itself did not have a name. The level editor was called Zero based on one of the earliest games it was used to build which was called something like Ground Zero at Activision but I don't know that it was ever released (this was in the 90's and would have been a PC game. ). The same engine was used on many games and modified independently for each one. In short the engine evolved and the code branched, features kept or discarded by each different team as needed. I think it was even updated and used for the Lord of the Rings game Pandemic made after SWBFII. Some of these games include Dark Reign, Dark Reign II, Triple Play, Battlezone (I think) Battlezone II, Star Wars Clone Wars, Full Spectrum Warrior and Mercenaries. They are all branches of the same game engine, all with different code and functionality but all branched off from the same code. The engine didn't have a name though, each game had a unique codebase that was modified to meet the needs of each game. Zero was just the name of the level editor. RedEngine I believe was the geometry model engine, Zephyr the animation engine, and Pebble(Pbl) the particle engine. Since they abandoned it for BF2 I doubt it’s readily available or that they have anyone who cares enough about it to invest their time (money) digging it out and having a lawyer work on a license for you. To release it even for personal use would require a license stating the terms of it’s use. I have a copy of what I think is the final BF1 code but I can’t release it and the code is obsolete so it does not compile. I’m not a programmer so I can’t update it to fix it either."
Zero Engine, to this day, has proven it is still a versatile engine with many more features possible with it discovered.