Admiral Ruskov has taken the liberty of giving himself a promotion. When he introduces himself to Captain Chandler he says that before the collapse of the Russian government he was a Vice Admiral in the Russian Navy's Red Banner Northern Fleet, which is the Russian Navy's Arctic Ocean fleet responsible for patrolling and defending Russian's northern shores along the Arctic Ocean, as well as the north Atlantic in the vicinity of Norway and Finland. A Vice Admiral in the Russian Navy has two stars on their shoulder epaulets and one thick stripe with two standard stripes on the sleeve. Ruskov is wearing four stars on his shoulder epaulets and has one thick stripe with four standard stripes on his sleeve, this is the rank insignia of the Admiral of the Fleet, the highest ranking officer in the fleet. There are only five Admirals of the Fleet, one for each of Russia's fleets: Northern Fleet, Pacific Fleet, Black Sea Fleet, Baltic Fleet and the Caspian Fleet.
IMO stands for International Maritime Organization which is a branch of the United Nations responsible for the regulation of maritime shipping in international waters. IMO numbers were introduced to improve maritime safety and security by making it more difficult for someone to impersonate another vessel. All civilian commercial vessels over a certain size are required to have a IMO number, specifically cargo vessels larger than 300 tons gross displacement and passenger vessels over 100 tons gross displacement. Once a number is assigned to a vessel that vessel continues to use the same number over the course of its life, even if the vessel changes names or ownership its IMO number would remain the same.
Whilst waiting docked, they received a radio call by a ship that introduced themselves as a British Navy Vessel, the H.M.S Suffolk. The crew of the U.S.S Nathan James verified the ship's identity, but there has not been a Royal Navy Vessel called H.M.S Suffolk since 1948, or more than 60 years. Thus the ship wouldn't have been verifiable due to its non-existence.
Clearly this is a case of the show choosing to use the name of a nonexistent ship to avoid confusion with any actual British warships.
Clearly this is a case of the show choosing to use the name of a nonexistent ship to avoid confusion with any actual British warships.