The U.B.C.S were first introduced in Resident Evil 3 as Umbrella's own privately funded army. They were sent in to Raccoon City to save civilians, completely unaware that Umbrella are the reason for the whole outbreak to begin with. The group is entirely filled with mercenaries and war-criminals who practically no ties to the company itself. As such, Umbrella see them as expendable and more than willing to throw them to their creations under the pretense of helping civilians, but with the true agenda of collecting battle-data.
The most well known three within the group are Carlos Oliveira, Mikhail Victor and Nicholai Ginovaef. Carlos played a supporting role through Resident Evil as a constant ally to Jill Valentine during the few days before Raccoon City's destruction. Mikhail only played a minor and was mostly seen lying on a tram seat because of severe battle-wounds. Nicholai, on the other hand, played the part of the human antagonist for the game. He was one of the ''Supervisors'' who were given a more detailed briefing of the operation, and were there to gather combat data as the U.B.C.S battled the B.O.W's.
Carlos would always survive the Raccoon City ordeal, as Mikhail would also always be killed by the Nemesis trying to protect Jill. Nicholai's fate, however is ambiguous depending on the ending. He could either be killed by the Nemesis, killed by explosion within his helicopter by Jill Valentine, or will manage to escape Raccoon City in the aforementioned helicopter if Jill chooses not to combat him.
Though the U.B.C.S haven't been encountered any further than the Raccoon City operation, they would still be found again in the Outbreak titles which are based around the said operation. Though Carlos and Mikhail aren't seen nor mentioned, Nicholai makes a cameo in Outbreak File 1 where he blows up the Raccoon City University and assassinates rogue Umbrella scientist, Greg Mueller. Multiple nameless U.B.C.S soldiers are seen throughout as well, mostly while combating Greg's own creation, Thanatos - a black Tyrant, basically.
They also appear during Outbreak File 2 as well, once again acting as B.O.W fodder. Another named U.B.C.S soldier is introduced, though, as Arnold. A selfish and cold marksman who's more interested with simply killing zombies than trying to save civilians.
Differences to the U.S.S
Unlike the Umbrella Security Services, the U.B.C.S are primarily intended for large scale assaults and skirmishes, while the U.S.S are reserved for more undercover procedures. Because of the U.B.C.S's roster of mercenaries and nobodies, they are seen as completely expendable and worth throwing to the undead wolves if it'll get them some needed combat data for their creations - direct or inadvertent.