What is the difference between complete and partial deinstallation of the ISPConfig system?
Partial deinstallation means that only the ISPConfig system itself will be uninstalled, not the objects created by it (web sites, users, DNS records, etc.).
On complete deinstallation also the objects created by the systems (web sites, users, DNS records, etc.) will be uninstalled. Thus, the server is reset to its original state.