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The U.S.S are most often always behind the scenes and are rarely encountered face-first within the story. They were a prominent group during Umbrella's reign within the series, however, and they are most well known for its notorious fan-favourite, HUNK. Thanks to the work of HUNK, Umbrella were able to gain William Birkin's developed G-Virus during Resident Evil 2.
During Resident Evil 0, under the orders of Albert Wesker who was still with Umbrella at the time, they had taken control of the Ecliptic Express which recently was unleashed to the T-Virus. All of the USS soldiers sent there are killed by t-virus imbued leaches, sent by a reincarnated James Marcus. As such they are in fact encountered as zombies fairly frequently during the game.
USS members are also apart of the cast in both Outbreak titles as well. Outbreak File 1 features an unlockable USS Agent skin, called Luke. Outbreak File 2 actually featured a story-related Agent in Rodriguez, who was also one of the first USS Agents to be shown without wearing the trademark helmet w/ gas-mask.
Operation: Raccoon City inferred the biggest exposure for the USS Unit, with the main campaign being centred around six different USS Agents--all who show different sorts of armor not seen before from the USS within the series--in a non-canon retelling of the Raccoon City outbreak during Resident Evil 2 and Resident Evil 3: Nemesis.
Differences to the U.B.C.S.
While the Umbrella Biohazard Countermeasure Service were the army of Umbrella, the U.S.S were the dark hand they reserved for more delicate operations. Unlike the U.B.C.S, U.S.S Agents are hand-picked and highly trained, in contrast to the rogue mercenaries simply plucked off of the streets or prisons that make up the general U.B.C.S roster. Umbrella actually saw the U.S.S unit as one to respect, unlike the U.B.C.S which they considered expendable and gladly would send out for combat data fully expecting them not to survive.