Antimatter tankers are a type of tanker designed to transport antimatter. Antimatter tankers were often escorted by other ships for protection against raiders. However, due to the highly explosive nature of antimatter, attacking an antimatter tanker directly was often seen as suicidal.
One type of Federation antimatter tanker was a boxy ship that resembled a large shuttlecraft with a complex docking nose. These tankers had a crew of about twenty plus passengers. They had three decks, the bottom of which was nothing but three cargo bays. One ship of this type was the Phoenix. (DS9 novel: Antimatter)