You define a hierarchy of device models that defines a set of variables and equations. A Device Model specifies a type of a device. The device model also needs ports that define how devices communicate with other devices.

A port also has a type, called a Port Model. A port model defines a set of variables that take part in the communication between devices. A typical example of this rather abstract description is an electrical circuit. All resistors in a circuit belongs to the same device model, the resistor model. The resistor model defines two ports representing the two pins that a resistor has. Each resistor is a device where the two ports to communicate with other resistors and possibly other types of devices like capacitors and inductors. The port model for the port needs a voltage and a current. The voltage represents the node potential of the connection, and is the same for all ports that are connected. The current that flows out of a port must flow into the other ports, so the sum of all currents must be zero. The port model declares this behavior.