Southside is the south side of Vice City in the edition text. The Ultimate Edition includes a raid on the PTT Youngin$ compound, described as one of the loudest gangs on that side of the city.
The full Ultimate text describes PTT Youngin$ as 'one of Southside Vice City's loudest and most socially active gangs.' You raid their compound and have to escape safely to keep the special items and contraband. It is one of the few places in VI that Rockstar has tied to a specific gang.