There is a lot of outdated and/or partially incorrect information about Fleets floating around. This guide is based on rigorous testing and up--to-date as of 2021.
The following information has all been obtained by constructing the relevant scenarios in the game editor and testing behavior.
- Fleet Postures only applies to enemies that you're at war with, not to pirates
- Fleets behave according to posture even if they are not automated. However, this is buggy for attack posture where the fleet can get stuck once the posture target is destroyed. This bug can be avoided by automating attack fleets. However...
- For automated fleets, the AI will pick posture attack targets and home bases (even if fleet formation automation is disabled). However, home base changes seem to happen only very rarely and not in any sensible manner, so maybe this is actually a bug and the home base is intended to stay unchanged.
- Fleet posture range changes for defense fleets by the AI do not seem to happen - but it's hard to provide a negative...
- Fleet posture (defend/attack) is changed by AI only if fleet formation automation is enabled (and only for fleets that are automated)
- Engagement stance is different from fleet posture. Engagement stance is controlled at the lower right corner of the unit status window and controls which targets are attacked by a ships. The default engagement stance when giving an order (attack / patrol / escort / stop) can be set under "Empire Options").
- Engagement stance applies to enemy units in the same system as the fleet (not necessary the home base). Fleet posture applies to targets outside of the system and acts on a slower timescale. When the fleet responds to something, the Engagement Stance of the fleet changes to "Engage detected targets". This means that:
- If engagement stance is "only when attacked", then the fleet will not protect targets in its current system - even if these targets are under direct attack.
- If there are enemy units outside of the current system, the fleet will attack them according to its posture range - no matter the engagement stance. However, once the fleet reaches the system, the engagement stance will apply.
- Therefore, the Engagement stance of a fleet should generally be "System targets".
- Attacking fleets can get separated if parts of the fleet go after escaping enemy ships. Automating the fleet ensures that it will reassemble afterwards.
- An automated fleet generally returns to its home system on its own.
- If a ship is part of a fleet, it generally stays with the fleet. If fleet is automated, manual commands to a ship are cancelled immediately. If fleet is not automated, you can generally give a manual order and the ship will return to its fleet when it has executed the order. But obviously, if you give an order to the fleet before that, this will take precedence right away.
- The difference between ordering a fleet to attack a target directly and giving it in attack posture with the same target is that with an attack posture, the fleet will *afterwards* attack other targets near the original one (according to its posture range).
- The difference between the "Attack" and "Prepare and Attack" order (which you can give to fleets) is that for the latter order the fleet will first spend some time waiting until most of its ships are in the same place before jumping to attack.
- If a fleet seemingly does not do anything at all, then it may be because targets are out of its reach (eg if a single ship in the fleet only has very little fuel left).
- If the engagement stance of ships in a fleet is constantly overwritten, then this is probably because the fleet gives the ships implicit orders, which every time re-apply the default engagement stance. The way to avoid this is to set the default engagement stance to "No Default Stance / No Change" for "Attack" and "Other".