"When picking a target, you won’t be able to shoot enemies that are in combat with other units, much like the current edition. However, you can fall back from combat in your Movement phase, allowing other units to fire at your opponent at the expense of your own actions this turn. Expect to see cunning generals deploying their armies in waves to take full advantage of this.
Heavy weapons are worth talking about too. These no longer snap fire if you move, and instead they have a flat -1 to hit modifier for moving units. This applies to all models with heavy weapons, vehicles included. There are a few other factors that affect hit rolls too – smoke launchers on a vehicle, for example, have the same effect of -1 to hit.
The last big change we’re going over today is cover. Currently, cover saves give a blanket save to all units, and one that only comes into effect if the shot would otherwise ignore their armour. In the new Warhammer 40,000, cover is a bonus to your armour save. Critically, this ability often only applies to certain types of unit. For example, only Infantry gain the bonus of cover from a crater.
This interaction works quite nicely with the modifiers to armour saves of certain guns, and means that when someone is trying to hide behind a wall or barricade, if your weapon has a high enough armour penetration, you can shoot them through a wall!
There are also a few weapons that ignore this bonus cover to armour effect – such as those wielded by Chaos Noise Marines and a Leman Russ’ nova cannon."