The Reactor Management Sector, or RM Sector in short, is the floor where the Control Room, the Grav-Shafts to the Heavy Maintenance Sector and other main reactor rooms are located. It is located above the Heavy Maintenance Sector. The spawn (note: not respawn point!) is also located on this floor. When you use the Gateway Transporter that you spawn in on the RM Sector, you will be transported to Heavy Maintenance Sector.
The RM Sector is split into 3 main subsectors, that being the Reactor Control Room, Reactor Management Facilities and the Main Reactor Chamber Access (note: leads to Grav-Shafts which will lead to the actual Reactor Chamber).