PTR records, or pointer records (also known as reverse DNS or rDNS records), map an IP address to a domain name. PTR records are used for reverse DNS lookups.
PTR records are the opposite of A and AAAA records, which map domain names to IP addresses.
We automatically create PTR records for Droplets if the name you give it is a valid FQDN. Droplets with IPv6 enabled only have PTR records enabled for the first IPv6 address assigned to it, not to all 16 addresses available.