OS X Server Caching / DNS Question
Reading the help pages I would assume that prs stands for public range comma-separated and prn stands for public range binary network-byte-order format.

The entry(ies) go(es) to the authoritative DNS-server which is not necessarily your (internal) DNS-server.

The purpose of the entries (of the public hosts or network ranges) in the zone file is to introduce your caching server and then redirect any of the service queries (e.g. updates) of the other hosts to your caching server.

Rough example:


Your public host             ---update request-->                            Apple Update Server
Apple Update Server      ---reverse DNS look up--->                 auth. DNS server
Auth. DNS server            ---domain name/_aaplcache._tcp---> Apple Update Server
Apple Update Server      - determines the responsible caching server
Apple Update Server      ---sends public ip of caching server---> your public host
Your public host             ---update request-->                            your caching server