There is three types of this route:
❶ "All Stop" service, which follows from Brooklyn through Downtown Manhattan to Midtown Manhattan and backward with all said stops.
❷ "Midtown Express" service, which follows from Brooklyn to Midtown Manhattan skips stops in Downtown. It serves on weekdays rush hours in the peak direction only.
❸ "Downtown Express" service, which follows from Brooklyn to Downtown Manhattan only. This servise was described on a separate page «BM2 (Downtown Express)».
Full information you can see here