The type of source identifier, such as DOI, ISSN, ISBN, publisher’s identifier, etc. for a <related-object>
Used along with @source-id to identify the publication that is (or contains) the target of the <related-object> element. The @source-id might contain an ISBN as content, and the @source-id-type should indicate that this ID is an ISBN.
|Text, numbers, or special characters||The type of identifier that names the source of the related object, for example, a DOI|
|Restriction: This attribute may be specified if the element is used.|