The "detach" command:
Change the project-code and make other changes to REPOSITORY so that it becomes a new and distinct child project. After being detached, REPOSITORY will not longer be able to push and pull from other clones of the original project. However REPOSITORY will still be able to pull from those other clones using the --from-parent-project option of the "fossil pull" command.
This is an experts-only command. You should not use this command unless you fully understand what you are doing.
The original use-case for this command was to create test repositories from real-world working repositories that could be safely altered by making strange commits or other changes, without having to worry that those test changes would leak back into the original project via an accidental auto-sync.