When a DNS server receives a recursive request from a client that it is willing to perform recursion for, it will go through the process of resolving the requested domain name by first asking the root servers, which respond with a referral to the top level DNS servers, then asking one of those servers, which respond with a referral to the next level DNS servers, etc.
When a DNS server receives a non-recursive request or a request from a client that it is not willing to perform recursion for, it typically responds immediately with whatever local data it has available at the time without doing any additional processing.
Four types of recursive requests to the local DNS sever in Plesk are available:
Note: Not all of the following types can be supported by the server. Before setting the recursion type, use the get-supported-recursion to make sure it is supported by the server.