What were "circuits" as noted in the biography, A Woman of No Importance?

The chains of connection between various agents of the French Resistance were known as "circuits". Virginia was connected with several circuits, while several members of each circuit had their own connections with other agents that Virginia did not know about. This relative lack of knowledge was one way of keeping Resistance operations secure and functioning; a captured agent may be forced to reveal some, or all, of what they knew, but because what they knew was limited by the circuit structure, they could not reveal everything.