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TouchDesigner’s parameter system allows for driving an operator’s parameters with another parameter. This is commonly referred to as parametric constraining. A dashed line with the arrow between the operators shows that there is a parameter relationship between those two operators.
There are two common methods used to drive a parameter with another parameter: Referencing and Binding. References rely on a python expression to evaluate the source parameter and update the destination or reference parameter. This is a one way communication. The Reference is denoted on the circle rotate parameter as a blue color.
Binding exhibits the same behavior as a Reference with the added ability for the destination parameter to control the source as well. This is a two way communication between the parameters. Binding is denoted on the rectangle as a small purple triangle in the parameter field and purple text on the circle.
Even though Binding is bidirectional, Referencing allows you to put complex expressions in a parameter.