Optional Destinations or Find Me/Follow Me?

Under Advanced, Optional Destinations you can specify where the call is sent if it is not answered. Under FindMe/FollowMe you can do more or less the same thing.

Can someone explain to me in a simple way what the difference is and in which case which option is better used? Thank you in advance.

The main difference is that the phone user controls when FMFM is enabled and where the call goes vs. the optional destinations are admin settings beyond user control.

The user controls that with the UCP, not the phone, right? However, most users are used to setting a call forwarding on the phone.

And is it correct that you can only specify extensions in FMFM, but no ring groups, queues, etc.?

You can put any digits you want in FMFM.