Check the docs...
http://download1.swsoft.com/Plesk/Plesk8.0/Doc/plesk-8-admins-guide/17359.htm
Or read here:
Including Directives into Web Server Configuration File
You can include domain-specific Apache configuration directives into web server configuration file. In Plesk each domain has virtual hosts configuration stored in a separate file httpd.include.
On all Linux systems, this file is located in the directory /var/www/vhosts/<domain-name>/conf/.
On FreeBSD systems, this file is located in the directory /usr/local/psa/home/vhosts/<domain-name>/conf/.
If you upgraded from Plesk version 7.5.4 or earlier, try looking for this file in the directory /home/httpd/vhosts/<domain-name>/conf/.
Note: you can change the location of virtual host directories using the transvhosts.pl utility, which is located either in /usr/local/psa/bin/ or /opt/psa/bin/ directory, depending on your operating system.
This file is overwritten each time the virtual host configuration is changed, thus any manual alterations made to the file are discarded. To use custom directives or redefine those inserted by Plesk, you need to create the files vhost.conf and/or vhost_ssl.conf with necessary directives in the directory /path_to_vhosts/<domain-name>/conf/ for a domain, and /path_to_vhosts/<domain-name>/subdomains/<subdomain-name>/conf/ for a subdomain.
If any of these files exist by the time the main configuration file is generated, Plesk inserts the appropriate Include directive into the HTTP and/or HTTPS virtual host context respectively. For security reasons, only root can create the vhost.conf and vhost_ssl.conf files.
For the changes to take effect, you need to run the following command:
/plesk_installation_directory/admin/sbin/websrvmng --reconfigure-vhost --vhost-name=<domain_name>
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