When creating a Plant 3D project and uploading it to BIM 360 or Autodesk Construction Cloud all project related paths and files (like spec files) have to be contained within the project folder. They cannot afterwards be re-referred to another (BIM360/ACC) location.
With a non-collaborate project the spec file folder can be referred outside of the project. This allows several projects to share the same spec files. In turn this makes it easier for a CAD-manager to ensure that all specs on all projects are up to date and in sync with the latest edits.
With a collaborate project this is impossible. An administrator cannot setup multiple projects and have them refer to the same spec files. Any update to a spec file that is shared by multiple projects has to be manually updated in several projects. Since there is no link to see which projects use which spec files this information has to be kept seperately from the Plant 3D environment.
Allowing collaborate projects to refer other folders/files outside of their project map (but maybe still within BIM360/ACC) would maintain the SPOT (Single Point of Truth) philosophy of BIM360/ACC.
As it stands now, in certain cases, a non-collaborate project setup turns out to be more SPOT friendly than a collaborate project.