From Paterva Wiki
This transform extracts the DomainEntity from a DNSNameEntity. The domain in a DNS Name like 'mx.google.co.uk' would be 'google.co.uk' and 'co.uk' (and 'uk' if you really want to be precise). Because these TLDs and subTLDs are really not that useful it is not returned.
Assume that you obtain a list of nameservers for a domain using the DomainToDNSName_NS transform. You want to investigate the resulting DNSName a little further. The more interesting organisation data would be located on the domain. The DNS name is converted to a domain for further investigation using this transform.