The NOIP protection code can be installed on all connections, or separately on synchronization.
The access code allows protecting all NOIP connections, or just NOIP Synchronization.
If you enable only the synchronization access code, ID Connections will be possible without an access code, if you enable access code on all connections, an ID Connection will require an access code both on Viewer and the server.
The access code is indicated on the NOIP Synchronization tab in the ID connection properties on Viewer and the server.