Sources (triangles pointing upwards) are Nodes that create Resources.
As any other Node in a Machinations diagram, Sources can be automatic (the default), interactive, passive, or activated once before a diagram starts, depending on its Activation Mode.
An example of an Automatic Source is the steady regeneration of the protective shields of the player’s star fighter in Star Wars: X-Wing Alliance. The action to build armies in Risk would be modelled as an Interactive Source of armies, and passing Go in Monopoly would be a Passive Source of money that is triggered by a game event.
In many ways, a Source acts just like a Pool without inputs that starts with a sufficiently large (or even infinite) supply of Resources. However, to model limited Sources, it is better to use a Pool with a specified number of Resources in it.