By default, Plesk uses the Apache web server (http://en.wikipedia.org/wiki/Apache_HTTP_Server) to deliver the pages of your website to clients (such as browsers, through which visitors access your website). To achieve better performance, Apache is supplemented with another web server - nginx. You also have the option to use nginx only.
Default web server settings are specified by the server administrator (hosting provider). These settings determine how web servers process various types of files, how they use SSL/TLS, where they store log files, and so on.
However, you (as a website owner) can set up custom web server settings for your website. For example, add a type of the index file, restrict access to the site, and so on.
Note: You can adjust web server settings for your websites if your hosting subscription provides the corresponding permission.
Apache and nginx have certain settings you can customize on the domain level. They are located in Websites & Domains > domain name > Apache & nginx Settings. These settings are divided into two groups: