The company is divided into two parts (series). The company will usually do training events together as a whole but often they will split into the series for training. For instance, Lead Series' platoons may do a certain training event in the morning and Follow Series may do it in the afternoon or vice versa. Sometimes they even divide up for whole days. They do this so some of the classes are not as big and the DI's can help the recruits out more or they jsut have a short amount of time to get something done and they would not have enough time to have the entire company do it all on the same day.
There is no significance as to which recruits get placed in which platoon. It is very simple. The first 75-80 recruits to arrive at Parris Island are placed in platoon xxxx and the next 75-80 recruits to arrive are placed in platoon XXXX and so on.