First need to know what is user assign thing. UserAssign is your userassign based on what type of drive or operating system you are using. If you are a os user and on top of that you are using  browser then os browser is your userassign.

A string that identifies the browser accessing a website is known as a useragent in JavaScript. The server uses this string, which is sent as part of the HTTP request, to decide which version of the website should be served to the user.