I have encountered with apparatus fontium for example in this reference:
Gundissalinus, De divisione philosophiae, apparatus fontium ad pp. 36 –7
What is it and what is it's the literal meaning?
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Apparātus is a very general-purpose word in Latin, but in classics and textual criticism, it refers to extra material that an editor has attached to a work; I'd translate it as "footnote". For example, an apparātus criticus explains how the primary sources differ for a given line.
An apparātus fontium, then, is a "footnote of sources". In other words, an editor has added a list of the sources that the original author refers to. If the author quotes Vergil, for instance, the apparātus fontium for that line will provide the book and line numbers.