Source literature is a term with different meanings. Literature (understood as printed texts) is one kind of information source. In a way, all literature is a kind of source literature. It might, for example, be cited and used as sources in academic writings. However, if used in this broad meaning the concept becomes synonymous with literature and the term thus superfluous and meaningless.
The meaning of "source literature" is relative. From the point of view of a bibliographic index the indexed papers are "source literature". For example, in the Social Sciences Citation Index is a "source index" covering the journals being indexed. These journals are the "source literature" from the point of view of this index. But from the point of view of the indexed papers are the bibliographical references contained in the single papers "source literature".
In the humanities, the term "source literature" has a more precise meaning as published sources: Many archives, for example, publish important sources to be used by historians and other scholars as reliable editions of formerly unpublished sources. The publishing of such sources requires knowledge of text philology and other fields. But this kind of expertise put into the publishing of source literature should be differentiated from the kind of expertise needed in order to use the sources in, for example, historical research. A historian may or may not use such "source literature" and on the basis of his research publish a paper, which in the UNISIST model is considered primary literature.
Fjordback Søndergaard, Andersen & Hjørland (2003) thus suggests that source literature is a distinct kind of literature to be distinguished from primary literature.
Fjordback Søndergaard, T.; Andersen, J. & Hjørland, B. (2003). Documents and the communication of scientific and scholarly information. Revising and updating the UNISIST model. Journal of Documentation, 59(3), s. 278-320. http://www.db.dk/bh/UNISIST.pdf