After the Decimation, initial estimates placed the number of surviving mutants at less than 200. Although the final number was actually closer to 300, the survivors latched onto the original number of 198, and adopted it as their personal banner. Most of the surviving mutants in America congregated at the Xavier Mansion for sanctuary and protection thanks to the X-Men. Unfortunately, they soon also found themselves under the supervision of the Office of National Emergency and their Sentinel Squad, who had taken an interest in "preserving" the mutant race even if it meant keeping them illegally detained on the Xavier property in a makeshift reservation. The 198 rebelled, and with the help of Iron Man they eventually secured the right to come and go as they pleased again in Civil War: X-Men #4.
While all mutants after M-Day could technically be considered part of the 198, this group specifically refers to those who lived in the camp on Xavier property after the Decimation.