What is j_security_check when you login to Assure?

The most common way of authenticating a user in web applications is through a login form. 
In Form-Based Authentication, Java EE security defines the j_security_check action for login forms. 
j_security_check is part of all standard JSP/servlet/J2EE container-based form authentication.

For Assure, the default URL for the web app is index.jsp which redirects to "authorize.sp" (or .do for older releases). authorize.sp will look up a user's stored preference for "home page" and redirect you there (inbox, results by control point, rules | controls, etc.). If there is no preference defined, you get redirected to the "welcomenewuser.sp" page.