What layer of an SDN implementation uses programs to communicate needs for resources via APIs?

The application plane of a software-defined network (SDN) is where applications run that use application programming interfaces (APIs) to communicate with the SDN about needed resources. The control plane receives instructions and sends them to the network. The last common plane is the data plane, which forwards devices handling data and takes inputs from the control plane. The monitoring plane is not a common layer in an SDN environment.