ACS Defend and HOLD both provide defensive support, but they are separate mission paths. In either case, the fleet has to be physically present at the correct target when combat begins.
How does this rule work?
Inter-War validates the relationship that authorizes support and checks target consistency. Support can be allowed for your own object, a member of the same alliance or an accepted buddy.
What exactly does the server validate?
From the battle perspective, the mission name matters less than whether the fleet arrived in time and is still staying at the target. A fleet that is already returning does not defend.
How does this affect the mission?
In practice, HOLD is useful for keeping a fleet at a target for a defined period, while ACS Defend provides organized defensive support. Both require careful timing and the correct target type.
When does it matter?
Defensive support is broader than offensive ACS: you may support your own target, a member of the same alliance or an accepted buddy. Permission alone is not enough—the fleet must also be on the correct object at the relevant time.
Common mistake
For defense, the state at battle start is decisive. A fleet that has been recalled or is already returning is not treated as present at the target. HOLD/ACS Defend timing therefore matters even when the mission was originally dispatched correctly.
Related FAQ: Does a returning fleet still defend the target? · Who can join an ACS attack?