This information is part of the Modelica Standard Library maintained by the Modelica Association.

Connectors PositivePlug and NegativePlug are nearly identical. The only difference is that the icons are different in order to identify more easily the plugs of a component. Usually, connector PositivePlug is used for the positive and connector NegativePlug for the negative plug of an electrical component.
Connector Plug is a composite connector containing m Pins (Modelica.Electrical.Analog.Interfaces.Pin).