The U.S.S. are usually kept behind the scenes and are rarely encountered face-first within the stories. They were a prominent group during Umbrella's reign within the series, however, and are most well known for its notorious fan-favourite, HUNK. Thanks to the work of HUNK, Umbrella were most notably able to steal the William Birkin developed G-Virus during Resident Evil 2.
While the Umbrella Biohazard Countermeasure Service were the army of Umbrella--one that is also largely unaware of Umbrella's actual affairs and motives--the U.S.S. were the dark hand they reserved for more delicate operations. Unlike the UBCS, USS 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 typical UBCS roster.
Appearances in Resident Evil
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 of whom are wearing uniquely designed styles of armor--in a non-canon retelling of the Raccoon City outbreak during Resident Evil 2 and Resident Evil 3: Nemesis.
While like Umbrella the USS no longer exist within the timeline, HUNK continues to make the odd cameo appearance in minigames throughout the series.