I believe this is done using IP rerolling. Basically; a DNS record is created by the domain’s owners to an IP address to verify its ownership. However over time, there might be reasons where the original servers loses the initial IP they were given. This is typically not a problem as long as the IP in the DNS records is updated. However if the subdomain stops being used via official means and people behind it does not delete their subdomain records from the DNS; A malicious actor could reroll their IP until they get the desired IP; and they can control that subdomain if they do.
Good, keep going that way.