In some circumstances, pages may need to be protected from modification by certain groups of editors. Pages are protected when a specific damaging event has been identified that cannot be prevented through other means such as a block. Otherwise, Wikipedia is built on the principle that anyone can edit it, and it therefore aims to have as many of its pages as possible open for public editing so that anyone can add material and correct errors. This policy states in detail the protection types and procedures for page protection and unprotection and when each protection should and should not be applied. Protection is a technical restriction applied only by administrators, although any user may request protection. Protection can be indefinite or expire after a specified time. The various levels of protection are detailed below, and they can be applied to the page edit, page move, page create, and file upload actions. Even when a page is protected from editing, the source code (wikitext) of the page can still be viewed and copied by anyone. A protected page is marked at its top right by a padlock icon, usually added by the {{pp-protected}} template.