This reddit post is old, and describes how flotillas fire on each other.

This is bonkers. There is no feedback in the game for any of this. There is no way of predicting any of this without reading this reddit post. This has severe implications for the entire battle system - sometimes more important than any other factor -  yet is completely counter-intuitive and undocumented, and presumably unintended (I am tempted to put this in the bug reports forum). It has been present for a huge amount of time. Is it ever going to be addressed?

On the Influence of Tactics on Cross-Flotilla firing (+some info on bombers)

In Endless Space 2, when can a flotilla target another flotilla? 

 

The widely known

A flotilla can target adjacent flotillas if there are no enemy ships in its opposing lane, and it will work as if the attacking ships fired from long range.

That fact is widely known. It mainly means that the Topmost Flotilla (Lane 1) will never be able to target the Bottommost Flotilla (Lane 3) and vice versa. (except if squadrons are involved)

 

If that were the only condition behind cross-flotilla firing, then that would mean the Middle Flotilla (Lane 2) should be able to target all Lanes in all circumstances right? Unfortunately,  that is not the case. 

Some of you may have noticed it during gameplay, but there are cases when a flotilla will be unable to target an adjacent flotilla.

 

Flight Paths, Range, Attacker and Defender...

The two culprits behind this apparent inconsistency are flight paths and firing arcs. Ships can't fire in their back, so a bad flight path can lead a flotilla to be unable to fire back at another flotilla.

Flight paths are determined by Tactic Ranges, and starting positions determined by who is the Attacker and who is the Defender. The Defender is "on top", and the Attacker is "under".

 

There are 6 possible opposing range combinations. In the following examples, the Defender's range is left and the Attacker's range is right. You only need to mirror the paths to get the symmetric combination.

 

Defender's Range - Attacker's Range

• Short vs Short (SS):

See Lane 2

The Defending Lane 2 (Short) can only target Lane 1 and 2. 

The Attacking Lane 2 (Short) can target Lane 1 and 2 at all phases, and Lane 3 during phase 1 and 2.

• Short vs Medium (SM)

See Lane 1 and 3

The Defending Lane 1 (Short) can only target Lane 1 and 2. 

The Attacking Lane 1 (Medium) can target Lane 1 and 2.

• Long vs Short (LS)

See Lane 2

The Defending Lane 2 (Long) can only target Lane 1 and 2. 

The Attacking Lane 2 (Short can target Lane 3 during phase 1 and 2.

• Medium vs Medium (MM)

See Lane 1

The Defending Lane 1 (Medium) can only target Lane 1. 

The Attacking Lane 1 (Medium) can target Lane 1 and 2 at all phases.

• Long vs Medium (LM)

See Lane 1

The Defending Lane 1 (Long) can target Lane 2 during phases 1 and 2. 

The Attacking Lane 1 (Medium) can target Lane 2 at all phases.

• Long vs Long (LL)

See Lane 2

The Defending Lane 2 (Long) can target Lane 3 at all phases and Lane 2 during phases 1 and 2.

The Attacking Lane 2 (Long) can target Lane 1 at all phases and Lane 3 during phases 1 and 2. 

 

Things to keep in mind:

-Lanes can almost always (99% of battles) target the Lane above them.

As an Attacker:

-Medium range will enable Support toward the lower lane, except if the Defender chooses Short range. But Medium will also prevent support toward the upper lane after Phase 2 if the Defender chooses Long range.

-Long range will enable Support toward lower lanes for phase 1 and 2, except if the Defender chooses Short range.

As a Defender:

-Short range will prevent opposing Support toward lower lanes, except if the Attacker chooses Short range. 

-Long range can prevent an Attacking Medium range from targeting the upper lane after Phase 2.

 

The above gives an important tactical value to ranges. For example, let's imagine two fighting fleets. Fleet A (Attacker) is evenly spread across lanes, and is going for full long range. Let's say fleet B has less ships, and would lose if it evenly spread its lanes like A.

Well, if B put a single ship in Lane 2, and the rest of its fleet in Lane 3, it could be possible to use Short range on Lane 2 to prevent Attacking Lane 2 from targeting Defending Lane 3. That way, Defender would lose 1 ship to then earn two third of the enemy's fleet despite the initial disadvantage.