IE emulation : difference between “Document Mode” and “User Agent String”?

Question

I would like to simulate the rendering (especially the responsive behaviour) of my web pages in the different IE versions. I am using the emulation fonctionnality in the F12 toolbar.
But I don’t understand if I have to use both “document mode” and “user agent string”. What is the difference ?
If I select a document mode = IE7 and let user agent string to default, I obtain a jquery error. But, if I select a document mode > IE7 and select user agent string = IE7, it works. So, what is the good way to simulate ?
IE 11 is installed. What is simulated if I select “Edge” ?
Why is there no IE6 in Document Mode ?

Answer 1

I found some explanations here : https://msdn.microsoft.com/library/dn255001(v=vs.85).aspx :

F12 tools in Internet Explorer 11 has simplified the complex matrix of browser and document modes from earlier versions into a single option: the document mode.

Six options are available. Edge (Default) represents all the latest standards and features IE11 supports. The other five options represent Internet Explorer versions 5 (a.k.a. "quirks"), 7, 8, 9, and 10. When you pick a version other than IE11, IE11 renders webpages as if you were using that version of the browser. It even changes the default user agent string the browser is sending. You can manually set a different string using the User agent string menu in the Mode section.

It's worth noting that the Document mode is an emulation of the older browsers. It can be really useful, but if you need pixel-perfect layout or to be sure your pages look and work a certain way in older versions of IE, we suggest going to Modern.ie and downloading their virtual machines with older versions of IE. The virtual machines work with virtualization environments for Windows, Mac, and Linux, so you get the most accurate and trustworthy platforms for testing and debugging in older versions of IE and Windows.