One of the tasks of Author-it Cloud Migration Toolkit is to check the database for all files used by the library that are stored outside of the database. These are your external files.
External files includes publishing templates, images used by the templates, and image files used for linked images. It also includes any other external files referenced by objects in your library.
Author-it Cloud Migration Toolkit does this by checking each object to see if it has any pathways to publishing template files, linked image files, or any other files listed in the object properties. The pathways are checked in all objects that can reference files, these include file objects, book objects, title page objects, index objects, media objects, style objects, publishing profile objects, and table of contents objects.