System Ids identify the source location of an imported object.
When importing objects into the repository, you can specify that Astoria create a System Id to reference the full path of the object on the file system from which you are importing.
If you import a different document (or the same document into a different folder), when processing the referenced documents and files Astoria recognizes any objects that it has already imported, and references the previously-imported object in the database (instead of re-importing it). This avoids duplicate imports of the same content. Note that the path on the client must match exactly for Astoria to recognize the previously-imported object.