Secret laboratories allowed the production of a random unit or structure affiliated with a specific country. However, they can sometimes give the production capability for a unit that can already be created.
Affiliation with a side does not matter as an Allied commander could obtain the ability to train Soviet units such as Desolators or Terrorists, or other Allied units like the Tank Destroyer or the Sniper. These buildings were the rarest tech buildings in the war.
In certain missions, the Tech Secret Lab gives a specific, pre-determined unit or structure depending on the mission.
Notes about the Secret Lab
There is a tidbit about the Secret Lab to keep in mind when using this on maps or capturing them. The game does not choose the unit the player will receive when the Lab is captured. Instead, the game decides which unit to grant at game start, and it does not change for the duration. Capturing multiple Labs does not yield additional units, and this is why it can be captured and the player will not receive an additional unit.