Mars City: Union Aerospace Corporate Division is the third level of Doom 3. It is the same level as Mars City, but after the demon invasion. In this level, the marine must return to the Marine Command, where he is then given his mission to link up with Bravo Team in Alpha Labs, then have to travel through Administration. This is where the demo ends.
Notable features of this level include:
Picking up and using a Keycard; the pick-up sound is the same as in original Doom.
Getting the Machine Gun from Z-Secs if you didn't pick it up in previous level.
The infamous mirror scene.
The first trip with a friendly Sentry Bot.
Passing in front of a closed Monorail Station door.
For cells of the form x/y/z, x corresponds to Recruit difficulty, y to Marine, and z to Veteran/Nightmare.
Items that can only be acquired by using console commands are excluded.
Storage Cabinet #017: Supplies - 347
Weapons Storage - 584 ; in order to get it, right after exiting infirmary, there is a corridor with a path blocked by flames pouring of a pipe, and a Medkit next to it. You have to run through it to access to a secret zone and get the PDA with this code, and then back, traversing the flame again.
If you stay in the starting elevator for a few seconds, you will hear shouting and gunshots.