Smoke Screen prevents troops covered in it from being targeted by all defenses (save for all Mines, S.I.M.O. and Microwav'r), although they also cannot attack (save for Medics, Dr. Kavan, Crystal Critters and Dr. Vitamin) or use any hero abilities.
At higher levels, or for Operations, one can use Zookas with Smoke Screens. It works the same like the Warrior, but they have a much higher damage output.
If your Troops begin to ineffectively spread out during an attack (e.g. a lone Tank moves up to a Cannon), but you don't want to move all of them with a Flare, it can be useful to cover the endangered Troop(s) with Smoke Screen until your other Troops can back them up.
Having some gaps between your Smoke Screens is okay because Troops will still be covered by smoke for about half a second after they leave it.
The Smoke Screen is good for approaching defenses that you cannot reach easily without getting noticed by the other defenses (e.g. getting to a far away Rocket Launcher when there are two Flamethrowers in the way).
This is mostly useful in Operations when you need a certain Defense taken out.
When your Troops are moving up to a defense that is dangerous against your strategy (e.g. Tanks up to a Cannon), Smoke Screen can be substituted for other Gunboat Weaponry and be used to cover your Troops until they are close enough and ready to fire at the defense in question. Once it clears, all Troops will attack at once, quickly eliminating the target. This works especially well when using Tanks, since they have high Damage Per Shot and their shots travel to the target quick enough so that they shouldn't get hit by the defense before destroying it - this works best if the defense has a rotating turret (such as a Cannon) that is pointed so that it will have to rotate in order to fire on your Troops; this way, there is a better chance the defense is destroyed before it fires.
Note that defenses with higher rates of fire - such as Flamethrowers - may still be able to attack your Troops before they are destroyed; however, these defenses will probably be destroyed before being able to do much damage (unless it's a match-up such as Riflemen vs. a Flamethrower).
The Smoke Screen will not block damage (defenses can still hit them unintentionally or via airborne/AOE projectiles) and also cannot prevent Mines from triggering.
If you choose to retreat, use Smoke Screens on your Troops to prevent damage to them while they are running to the Landing Craft to minimize casualties.
If a troop is low on health and is getting hit by defense(s), place a Smoke Screen on it so that the defense(s) switch targets/stop targeting it.
The higher the level, the longer it lasts. Each upgrade increases the duration of the Smoke Screen by 1 second.
It has no visual differences in upgrades.
The Smoke Screen has sufficient duration to resets all troops' attack cooldown. Therefore, when the Smoke Screen wears off, all troops that were under it will attack in sync.
Smoke Screens have been used for years in both land and naval warfare.