The Buster Call is the ultimate form of military attack used by the Marines. It is a result of the Marine doctrine of Absolute Justice.
The target of a Buster Call is usually to destroy anything from a criminal or group of criminals to an entire island that has proven to be extremely dangerous to the World Government. So far only two islands have been known to have faced a Buster Call: Ohara and Enies Lobby, with Nico Robin and Aokiji present at both. Both islands were utterly destroyed from the firepower.
Since the Buster Call is an order of such destructive powers, only the very highest authorities are granted the power to initiate it. This authority is granted to at most ten people.
Four of these are the three Admirals (Kizaru, Fujitora, Ryokugyu) and the Fleet Admiral (Akainu): the highest-ranking authorities in the Marines.
The other six are their counterparts in the World Government: the Commander-in-Chief (Kong)
And the Five Elders (Gorosei).
Once one has the means and the authority, the actual procedure of initiating a Buster Call is extremely simple. The admiral, world leader, or designee simply presses the button on a Golden Den Den Mushi to summon a Buster Call to their location.
After the signal is received, five vice admirals and a fleet of ten ships nearest to the location of the caller are scrambled to that island.
Once the fleet arrives, they fire an initial salvo to effectively dial in their coordinates, locking in their ordinance to triangulate their fire upon the target. Then the Vice Admirals, as well as the entire fleet, lay waste to the island, annihilating everything in sight with impunity, regardless of who is still on the island. For example, during the Ohara incident, the refugee boat containing all of the island’s normal civilians was destroyed without prejudice, along with everyone inside, in case a historian had slipped on board. The Buster Call fleet even kills government officials if they are still left stranded on the island. Even Admiral Aokiji was shocked by the ruthlessness of the attackers.