I just don't get it.

If I decide to fight at Long and enemy decided to fight at Short does it mean thet we both will fight at short? I mean if he decided to fight at Close then he will be closing to me so we can't talk about Long here for me. Is there any explanation of these things?

There is an explanation of how battles work by Frogsquadron somewhere in the developer posts. I can't seem to root it out right now, but maybe this will help:

Explanation

Battles work in three phases where distances are closed between flotillas.

If the choice of range is the same, then the flotillas will arrive in those positions and remain in the same position. So let's say your middle flotilla is set to medium range, and the enemy flotilla is set to medium range. They will both arrive at medium range and remain at medium range for the whole battle.

If the choice of range is different, then the fleets will close one position per phase. So if you chose short range and the enemy chooses long range, then you will both start at long range in phase 1, then medium at phase 2, and then both be at short range in phase 3. 

For completeness, if you chose medium and the enemy chooses long, you will start at long range from each other in phase 1, then close the gap to medium for phase 2 and 3.

Flotillas always target their mirror flotilla first, and will target other flotillas once their mirror is dead. This targeting is always considered long range, and only weapons that are capable of firing on that side are used. So weapons on the left side of your ship won't be able to fire at flotillas on the right.

Finally, the number of flotillas affects morale. If you have 3 flotillas and the enemy has 2, then your weapons and armour will be more effective. The effect is higher if you have 3 and they have 1. You can also make this higher using some tactics cards.

Trade Offs

If you choose long for long range designed ships, you will always start in the optimum position. Long range weapons of equal level tend to have less DPS overall than short range weapons, but the pay off is that you get to use them optimally straight away with the intention of inflicting damage before an enemy can close on you. As all long range weapons work optimally at medium range as well, this gives you at least two phases of optimal fire. It also allows you to target other enemy flotillas if the mirror flotilla is destroyed.

If you choose short you have the disadvantage of potentially being destroyed before you can fire optimally, or even at all. Short range lasers can only fire at short range, but also put out huge amounts of damage. If you live to close the gap, then you will cause havoc. Short range kinetics are less powerful, but can fire all the time and have the added utility of of being able to target enemy missiles reducing any incoming damage.

Medium range allows you to negate optimal long range fire faster, but also leaves you vulnerable to being closed in faster by short range damage dealers. 

If all ships are super specialised, then the rock paper scissors version goes as follows:

Long counters Short (short can't optimally fire for 2 phases).
Short counters Medium (short can inflict huge damage for 2 phases)
Medium (soft) counters Long (medium avoids optimal damage from long for 2 phases)

Ship Design Strategy

Flotillas designed for short range need to have survivability in order to make it to the short range phase, as well as negating any short range damage from equivalent short range flotillas. This makes putting support ships designed to tank a necessity for these flotillas. Support ships are designed to have a greater targeting weight (i.e. will be shot at first) so should be outfitted with armour, shields, and short range kinetics to fire at enemy missiles. You should also use smaller ships in higher numbers. This is because smaller ships have higher evasion, and it means that your damage is spread across multiple vessels meaning it doesn't matter as much if you sustain a few losses. If you use a single large ship for short range, if might be tankier and more damage, but if it does die then you lose 100% damage.

Flotillas designed for long range are the opposite. Focus more on damage rather than surviving. You will still probably want some support ships to tank a little, but your primary goal should be to inflict as many casualties in the first phase as possible. Make use of larger ships as they are innately more survivable and have more weapons modules allowing you to fire more in the first phase. Having some high alpha damage flotillas can create a morale snowball. If you wipe out the mirror flotilla with your LR flotilla, then not only will you increase the effectiveness of all your ships, but you will also have extra fire from your LR flotilla.

Medium range ships should be more rounded and less specialised as they have to prepare both for the possibility of dying to short range, and getting peppered early by long range. If you prefer to build more general ships, then focus on medium range tactics cards.

Lasers vs Kinetics

Lasers fire looks more damaging, but it is also more negated by shielding. Have a peek at the advanced battle overlays both before and after battles to figure out how much shielding the enemy is using. If it's a lot, then switch to more reliable kinetics. If it isn't a lot, then use lasers. Always use short range kinetics on support vessels so they can target enemy missiles. Short range laser beams can only fire at short range, but are terrifying if they get to fire, even against shielded opponents. Here are a set of general rules:

Missiles: Use on specialised long range ships.
Lasers: Use on generalised long-medium range ships.
Kinetics: Use on generalised medium-short range ships/use on support vessels
Laser beams: Use on specialised short range ships.

Caveats

Much of the above assumes relatively even fights between fleets. However, the big determining factors on who wins a fleet battle are number of ships, how advanced those ships are, and whether the fleet is supported by a hero. A well designed fleet with impeccably executed tactics will still probably die against a more generalised, but larger and more advanced fleet.

Wow, thank you! That's pretty complicated system.

This should be pinned! 

It is both clear and complete! 

It answered all of my questions and the possible follow up a big kudos!

Perhaps this was the Frogsquadron post you were refering to. 

https://www.games2gether.com/endless-space-2/forums/65-general/threads/18857-es2-gdd-4-battle-overview?page=1

Yeh, that's the post. There have been a couple of changes since my last post, mostly that beams can now fire at mid range, albeit with a longer cooldown between firing. The same rules still apply generally.

Some people have been criticising shields saying that they are not very effective. I haven't found this to be the case. I found they are still very effective against lasers, and they also mitigate damage to your hull meaning that you need to make less repairs. Usually you'll want a mix of defence modules, focussing on armour for HP, with one or two shields. There are also more hero skills focussed on shield absorption and hull penetration. So fleets led by these heroes will often have really good armour piercing and lots of shield mitigation, so shields often scale better. Feel free to test it for yourselves, but you might want to deck out your fleets according to your hero skills. To give an example - Craver heroes get +40% to physical damage (kinetics/missiles), Sophon and Vodyani get +40% to energy damage. So you may want to outfit your ships to make the most of these skills. Try to make sure all your main fleets are hero led. Fleets led by heroes are considerably stronger.

The other change is fighters and bombers. In theory, bombers are good against large ships, fighters are good against bombers. They currently need balancing. Bombers are ok, but not really worth the slots, and fighters are nigh on useless against bombers. I'd hold off on getting them for now until they've been improved. Kinetics currently counter bombers as well, so all the more reason to use kinetics on support ships. 

On the shields effectiveness, I think there is some kind of bug reported where they don't regen between phases. So that makes them less effective than they should be.

Ah fair enough. Do you have a link to the bug report? 

I'm just reading up on interesting topics. I noticed this thread is from 3 months ago, but I'm wondering if there's been any changes?

Bucketsmith wrote:

I'm just reading up on interesting topics. I noticed this thread is from 3 months ago, but I'm wondering if there's been any changes?

I believe so yes. This thread was ongoing also while the devs were testing combat balance changes in a side mod called G2G Blanace mod. There have been adjustments plus the stuff that came in the beta preview update 1.15 I think.

How dose fighters and bombers factor in now? Are they considered long range?

Echoing the previous post - is there a more updated version of this?