Data source is an umbrella term for any piece of data attached to a Parameter or Parameter driving its value. It could be data that is animating a parameter (AnimationSourceItem or AnimationSourceItem) such as an Expression or Expression, StaticSource or StaticSource, Constraint or Constraint or an FCurve or FCurve. It could be an Operator or Operator. It could be a Shader or Shader or an ImageClip or ImageClip used on a texture projection or in the FxTree.