Advanced cluster missiles were improved versions of the cluster missile weapons system, boasting an improved exposive potency. They can be fired in groups of six.
Shortly after the destruction of the Echo Base shield generator, Luke, after killing a snowtrooper and manning the snowtrooper's tripod gun to fend off Imperial forces, fired on an AT-ST container and destroyed it. He then recovered the case of advanced cluster missiles that were within the destroyed container.
During Rogue Squadron's infiltration and liberation of the Maw Imperial Prison Facility, Rogue Leader Wedge Antilles, dropped various proton bombs from his BTL Y-wing starfighter on an environmental dome, and then procured some advanced cluster missiles amongst the rubble.