If your division is part of the HQ system and you are a resident member in the division, you will be asked to choose a Home vACC when first logging into the HQ. A vACC (virtual Area Control Center) manages an area of airspace within the division. Your Home vACC is where you will do your training and spend most of your time controlling in.
There may be some vACCs in the list that you will not be able to choose if they are "open". An "open" vACC is one that has no staff members and will be difficult for you to find help or training in that vACC. You will only be able to choose a vACC that has staff who will guide you in the training processes and in understanding the VATSIM network.