A source [Digital Artefact](siocactions:DigitalArtifact) which is used by this [Action](siocactions:Action) as a basis or point of reference.
A source is [used](siocactions:uses) by an Action but not [modified](siocactions:modifies) by it.
specifically links this property to Actions which [create](siocactions:creates) something, but presumably Digital Artefacts can be sources for other kinds of Actions as well.