Source texts are articles that are or contain copies of other texts, licensing permitting.
These may or may not include original encyclopedic article content. For some texts, an introductory section helps orient the reader, give important context, and match the style of other articles. The text may be annotated to some degree, often with internal links (chiefly for texts that didn't originally include links) and footnotes. It should be reasonably clear what is the original text and what is annotation.