"does not overwrite existing links", "which will clear all links" - I'd recommend to pick a single wording (URL in this case) and stick with it. Calling custom URLs "links" is confusing as it's no longer clear if those statements refer to URLs or another feature. Are there any means to review all existing custom URLs for a notebook? If not through the UI, can they be retrieved via the API?

Thanks, Fabian; revised to use "URL" instead of "links". We do not appear to have a way to review all existing custom URLs at this time.