The Registration Data Access Protocol (RDAP) is a publicly accessible database that can be compared to an international phonebook but for website domains instead of telephone numbers.
In the database, you can find information about a domain, for example who registered the domain and when the domain name is set to expire. (This type of search used to be called a WHOIS-search. )
When you activate domain monitoring, Semonto will access the RDAP database to retrieve the most recent information about your domain.
Some domains, for example, the ones ending in .be (Belgium) and .nl (The Netherlands), do not allow this kind of monitoring. This overview shows you all the domains that are supported. If you have enabled domain monitoring in Semonto and the test failed, please check this list. If your top-level domain (= the part of your domain name after the last dot) is not in the list, that could explain why Semonto could not perform the test.