Under Hosting settings, we see "Additional write/modify permissions"? What kind of additional permission given in this option?
The Additional write/modify permissions allow you to enable the write/modify permissions inside the httpdocs folder of the domain. These permissions are required, for example, if subscriber’s web applications use a file-based database located in the root of httpdocs or httpsdocs folder. More information on this permission can be found in Plesk documentation Permissions | Plesk Obsidian documentation
