Let’s think about a situation where you have a subdomain and you want it to be able to access files from the domain httpdocs directory. You cannot do that by default in Plesk, because of open_basedir. You can edit the httpd.include file in:
/var/www/vhosts/domain.com/conf/httpd.include
but that will only work until the next plesk restart or major modification.
But, [...]