If the key does not exist in the registry, it will be created. If it exists, the data with certain name under this key will be created or modified. Nothing will be deleted.
For example, the line [HKEY_LOCAL_MACHINE\SOFTWARE\SVNScc\SVNScc] specifies path to SVNScc key in the registry. If it not exist, the key will be created. Then the data named SCCServerName and SCCServerPath will be created (or modified if exists).