Isolation has two purposes. One, to keep the survivor away from anyone that she might, in the mind of the primary aggressor, abandon him for. It is not unusual for primary aggressors to try to keep their partner from all males. Two, isolation keeps the survivor away from anyone that may influence the survivor to leave, use boundaries, or respond to the primary aggressor differently. This often includes her entire family, and most of her friends. Often isolating actions will be rationalized by the primary aggressor stating they don't like the person. But isolation is about controlling the survivor's access to the person she likes or from whom she receives support.
Isolation can be directly demanded, but isolation may occur over time as the survivor seeing friends and family members so upset the primary aggressor that it is just two hard to maintain contact. The survivor will often have the choice of doing something socially with the primary aggressor, with the risk of being embarassed by irritable behavior or just not doing it. Attending social events without the primary aggressor is not usually possible, because of jealousy.
Frequent geographical moves also serve to isolate the survivor. This is increased if the survivor moves out of her home state and very increased if she moves out of her home country. A underappreciated aspect of such a move is that later if there are children, the survivor separating or divorcing will not be able to take the children back home where there is family and support without the agreement of the primary aggressor.