@licensor

The attribute licensor specifies one or more <agent>s who hold the rights over the material specified by the parent element (either the data of the current file, or of the source that forms the basis for the data).

Nothing should be inferred from a missing @licensor from <rights-source-only>. Its absence does not mean that the rightsholder is unknown or nonexistent. For more, see the section called “Rights and Licenses”

Formal Definition

Defined at: TAN-core.rng