Arbitrary sources generate an output voltage or current that is the result of a mathematical expression, and, in some types of Arbitrary sources, of further processing.

The ablity to describe both non-linear behaviour, using a vast amount of functions and operators, and dynamic behaviour, using differential functions or special source types, truly makes this collection of devices "arbitrary". They are the essential building blocks of macro and behavioral models.

Their advanced functionality obsoletes the limited functionality provided by the specific controlled source devices (for example, voltage controlled voltage source), which are primarily supported for legacy and compatibility reasons.