Synchrotron syncs package from Debian into the appropriate suites. E.g. Debian testing is synced to landing. This syncs source and binary packages. If a package in PureOS had downstream modifications (indicated by a pureos in the version number) it is excluded from the sync. Also packages than vanish from Debian are removed if possible.
This is useful if one wants to remove a downstream forked package for example;
sudo -u dak synchrotron sync unstable landing main <package name> --force
This command would force a sync between PureOS landing's "main" suite with Debian unstable.