No. ACS validates not only coordinates but also target type. A planet and a moon at the same position are separate objects, and a joining fleet has to match the leader’s selected target.
How does this rule work?
Inter-War checks whether the group is attacking a planet or a moon. A joining fleet using the other target type cannot change the nature of an existing group.
What exactly does the server validate?
This matters in systems with active moons where both options can appear under similar coordinates. The wrong target type can block participation even when galaxy, system and position numbers are correct.
How does this affect the mission?
Before joining, verify the object icon and type rather than looking only at coordinate numbers. This is especially important when launching quickly from Galaxy or managing several operations at once.
When does it matter?
ACS does not create a separate, looser combat system. A joint attack still uses normal player protection, attack limits, target validation, fuel and fleet-slot rules. The group only synchronizes several valid fleets into one battle context.
Common mistake
Before joining, verify the leader, target, object type and planned battle time. Most problems come from trying to join after the target changed, from outside the required alliance, or with a fleet that cannot arrive at the required time.
Related FAQ: Can you change the target of an ACS group? · How does ACS arrival-time synchronization work?