Simply put, if you put your cursor on a square, one of three symbols might appear on the tile's side. Half-shield, Full shield, or a wall.

Either of those shield symbols means, you are somewhat covered from that side, AND you can see and shoot over it. When you're behind a cover with shield symbol, you'll crouch behind it and automatically stand and shoot whenever you attack.

Wall means you are completely hidden from line of sight from that side. Both you and enemy can't see through wall, and can't shoot through wall. BUT you can still shoot enemies that are in line of sight and in range of your gun. When you hide behind a cover with wall symbol, you will hide behind the wall and step one tile sideways and shoot whenever you attack.

If there's no obstruction between a shooter and the target, it will ALWAYS hit. If the target is in adjacent tile, it will also ALWAYS hit (in other word, melee attacks and point blank shots always hit).

Cover objects will still somewhat protect from the 100% hit chance even if you're not actually directly beside it. In other words, there needs to be an actual, clean line of sight without any cover objects, for a shooter to get 100% hit chance (not including the cover they're hiding behind, since they will stand to shoot over it).

It is sadly quite hard to distinguish which objects are which and what side they protect from and whether something count as clean line of sight in the game, so there's nothing much to do other than gaining experience in real combat situation.

Healing and repair items are quite limited in the game. They won't get replenished between adventures, and there's no way to buy them between adventures. But there are some things on your side.

1. Anyone can use healing/repair item in travel screen. In other words, character with healing/repair skills only mean they can equip healing/repair item in their item slot, and can heal in combat (which is by no mean a small help).

2. You can heal/repair in between dialogues. That means, you can travel to a node, to see if there's a camp nearby. If it turned out the node is a combat node, you then can press 'i' or click your inventory icon in the pre-combat dialogue/story window to heal before combat.

There are quite some special actions, and they need bravery to use. They tie to your armor/weapon, in other words everyone who wear light armor or use a pistol have the same special actions. Most are quite self-explanatory, but there are some important things to note.

1. Killing enemy, healing, repairing, and evading, increase your bravery. In other words, when you wear medium armor, you can use one bravery to increase your dodge rate significantly, then when the enemy shoots you will dodge and regain the bravery back. When a character have healing/repair skill, they can heal/repair themselves to regain bravery, then use whatever defensive skill they have if necessary.

2. Ambush stops enemy in track. Sniper rifle damage is for some reason quite low, but characters with sniper rifles are the only ones who can do the equivalent of "overwatch" from XCOM games. And when an ambush activates, it will basically make the ambushed enemy skips one turn. A lot of the time, simply shooting an enemy with the sniper rifle is the better choice, but you can imagine that there's a lot of time when stopping them from doing something is more important (especialy since you will also damage them if the shot hits).