I'm not sure what you want to know based on the first 2 questions.
It works just like any other tld. PTR records are put under ".arpa" in the in-addr.arpa zone
What will happen if there is no PTR record exists????
Generally nothing. Its not required to have a PTR record. Sometimes mail servers will perform a reverse lookup on the sending IP. Some ssl sites require reverse DNS, and you may see a slow initial connection when using ssh if there is a lack of a reverse mapping.