Document Mode in Internet Explorer
The Document Mode selection lets you choose how Internet Explorer interprets the page, and can be useful for diagnosing compatibility issues. There will be a (Default) next to the mode being used by the page. You can choose another mode, the number indicates the version of Internet Explorer. Each mode makes a series of changes to the browser's behavior so that it closely emulates the older browser version. The page reloads when you choose a new mode so that the web server and client-side markup is reinterpreted in the new mode.
Changes made to the Document Mode in F12 Developer Tools apply only for the duration of the browser session; when you close the browser and return to the site it will be in the default mode again. If changing the document mode fixes the problem, the modern.IE scan can help what's causing the problem.
How do I make a document mode change permanent for a page or site?
Users who find that a site renders properly after being changed to a different document mode should report the problem to the owner of the web site. Microsoft also maintains a Compatibility View List which it can use to select specific document modes in order to solve compatibility issues that have not yet been addressed by the owner of a site; Users can report problems directly to Microsoft for potential inclusion on that list.
As a developer and owner of a web site, you can permanently choose the Document Mode used by all visitors to the page by including a special meta tag or HTTP header to the page. See Defining document compatibility for instructions on how to do that.
IT Pros can also use Group Policy to manage which sites render using IE7 or Quirks Document Modes, without making changes to the site itself. See Manage Internet Explorer 11 compatibility using Group Policy for more information.
Users can update and manage their local Compatibility View list using the instructions here: Fix site display problems with Compatibility View
Can I use document modes instead of testing new sites in older versions of IE?
Document Mode is only an approximation to the actual behavior of older versions of Internet Explorer, and should not be used to determine whether a site works properly in an older browser. Instead, developers should test with actual copies of the browsers they wish to support. Microsoft makes free virtual machine images available across platforms and can be downloaded at Virtual Machine Downloads.