What is classless subnet delegation?

Classless subnets are delegated in the reverse zone of the parent that is delegating.
NS records declare the existence of the subnet's domain name servers.
PTR records attach canonical names to reverse lookup addresses.
CNAME records can be used to create aliases for simplification.
The ISP uses one CNAME record for each reverse IP address in its reverse zone.

example:
2.0.168.192.in-addr.arpa. CNAME 192.168.0.2.example.com.
The 192.168.0.2 attached to example.com is an arbitrary label, chosen in this case to match the IP address.
The customer will simply need a PTR record to resolve 192.168.0.2.example.com to an IP address.