By default, DNS records for subdomains in Plesk are included in the parent domains' DNS zones. You can configure Plesk so that each subdomain has its own DNS zone. This is useful, for example, if you want to use a different name server for a particular subdomain, or if you want to decrease the number of DNS records in the parent domains' DNS zones by moving the records belonging to subdomains to separate DNS zones.
To give every subdomain a separate DNS zone, run the following command (this will affect all hosted domains and all newly created domains):
plesk bin server_pref -u -subdomain-dns-zone own
To turn this feature off, use:
plesk bin server_pref -u -subdomain-dns-zone parent