[caf]

.arpa is used for reverse DNS lookups. The PTR record for the IPv4 address 1.2.3.4 is looked up at 4.3.2.1.in-addr.arpa.

[brokenparser]

And for enum lookups, e.g. [phone] becomes 8.7.6.5.5.5.5.1.e164.arpa.