Lets you craft combat units, they will automatically launch, defend your base and attack enemy units. They are not intelligent, but they are powerful.
Mods introducing new content into the game.
New ways to deal with enemies, be it attack or defense.
I have the error message: Error in assignID, item with name 'piercing-rounds-magazine' does not exist.
displayed on launch after installing unzipped "Combat_Units_0.1.3.zip" in the mods folder
sounds like you don't have the right version of Factorio. Update to latest stable (0.13.20 I think, now)