I'm not sure of the particulars of how Fedora 4's projection feature works, but conceptually it goes something like this:
- Start with a pile of content that is managed outside of Fedora
- An admin configures Fedora 4 to become aware of one or more directories on a filesystem to which it has access
- Fedora 4 crawls the directory, creating "nodes" within the repository that represent the folders and files it finds
- For each file that is found: a persistent, unique identifier is minted; an initial version is created (if configured); a checksum is generated (if configured)
- At that point, the directory is still managed outside of Fedora, but it's also accessible by Fedora. If the other system is editing or writing to files in that directory structure, Fedora will need to occasionally be made aware of these changes to regenerate checksums, create new versions, and so forth.
So, Fedora 4 could point at another app's file/folder structure, yes!