Cover only affects hit chances.
(The single most important thing to consider on impossible)
Objects in the game affect Line of Sight.
Aim is the base hit chance. 65 Aim = 65% hit chance.
SCOPE increases, sectoid mind link, overwatch decreases it,...
Cover adds to your defence stat. Defence reduces the enemy hit chance.
20 Def = -20% hit chance. Remember that cover only reduces it aslong as the unit takeing the shot did not flank you.
Range then has a final effect on hit chance this can be positive or negative depending on the weapon. Shotguns positive hit close negative long range, snipers neutral long negative short.
Hunker down doubles the cover bonus! (cover not defence eg smoke grenade or ghost armor)
While I read many people consider cover broken inaccurate etc. I find that the game treats this very very precise except people consider aliens aim as crappy as your soldiers. They dont especially mind linked and on impossible they hit hard.
Hunker down as such should not be overlooked.
Defence and LoS is the only way to avoid damage health just lets you soak up more before you die. Big hint for all that try impossible and find they fail.
Now the numbers you asked for:
- Partial cover +20 defence (Cover type)
- Full cover +40 defence (Cover type)
- Hunker down doubles cover type defence
- Being flanked removes cover type defence
- Smoke grenade adds +20 defence or 40 if skilled. (Normal defence)
- Various armor adds defence (ghost +30 Normal defence)
- Normal defence apply's even when flanked
I have not bothered with counting exact squares for optimal ranges yet. There is a very very limited weapon base in this game and you will get a feeling of all of them soon enough. Getting shot? Hunker down stop shooting those crappy low hit overwatch shots at dashing aliens ;) Disagree? Talk when you never lost a man on impossible :P
It seems that some enemies are tied to some missions, while others will appear anyway, but it seems that if you are quick enough they will appear earlier after certain missions.
Credit to yx and Sterno in the comments below !
Enemies which are tied to missions
Outsiders - First appear in a downed UFO. One appears in most (but not all) UFOs after that.
Sectoid commanders - first appears during the assault on the alien base. After this point, Sectoid commanders appear on the bridge of alien UFOs, replacing the Outsider. After the overseer ship mission is complete, they are replaced by Ethereals. Sectoid Commanders may also appear on harder alien abduction missions, usually in small groups. Source
Ethereals - First appear in the assault purple ufo mission, and then randomly in missions afterwards
Enemies which will appear anyway
- Sectoids
- Floaters and Heavy Floaters
- Mutons and Elite mutons
- Berserkers
- Sectopods
- Thin men
Spartacus mentioned that Chrysalids may be tied to the first terror mission.
Best Answer
According to this Source, "Electro Pulse is an Area of Effect ability that deals 5 damage to all units (excluding the MEC) within its radius. Robotic enemies are stunned for one turn."
It does 5 damage to all units (including allies but excluding the MEC using this ability in the sphere area around the MEC), then it stuns as well Robotic units for 1 turn. Here's a picture of affected tiles.