Some military units in AOE III DE can use Cover Mode. It believe it gives additional 50% resistance for both artillery and range attacks. It also decreased unit speed and inflicted damage by 50%.
However only some of these information are properly visualized in the game.
Let’s look into picture below with Pikeman in Cover Mode:
We can clearly see that speed and hand attack are halved as it should be (2.5 instead of 5.0 and 4 instead of 8), but we still see 0.10 Melee Resistance instead of 0.50 Siege and 0.50 Range Resistance.
This is a detail and experienced players knows what exactly Cover Modes does but at least for a beginners it should be displayed correctly. For now it looks like Cover Mode only takes something but gives nothing in return.