A source represents the function of a system serving as an infinite reservoir of mass or energy. No physically realizable has in principle unlimited capability to deliver mass or energy. However, the source function is used to provide an adequate abstraction of the physical phenomena considered.
1: A source has one output
2: A source has no input
3: A source must always be connected to one transport
4: A source must be related to a transport with an influence relation
The source function can only be used as shown in the figure below.