The USERID & PASSWORD combo does about the same as the NTNAME field with these differences:
USERID & PASSWORD will force QlikView to take over authentication but only for this document. You will be presented with a login dialog whenever you try to open the document (AP or QV Desktop).
NTNAME relies on Windows entriely for doing the authentication. This must have been done before, otherwise you wouldn't have access to Windows resources. If you approach a QlikView AccessPoint from outside of a Windows domain, you will be presented with a login prompt by your browser as instructed by the Windows server that is running the web server (the authentication solutions are more diverse, but that's not important at the moment). This is usually called SSO because logging in once in a Windows network may give you access to various network resources without logging in again.
If you use these techniques together, you will have double security (Windows account AND local UserID) but the end-user of your document will grow tired of having to log in over and over again. So do not do this unless you have good reasons to do so. And Row level security is not one of those, as you can implement row-level security just the same with just NTNAME and SSO.
BTW where did you read that story about row level security?