When the Prevent Multiple Sessions checkbox is ticked on the User Password Rules screen, this prevents the multiple users to be logged in at the same time with the same User ID. When enabled, the first user will be automatically logged out when the second user logs in with the same User ID.
Users with this security feature turned on will not be logged out or receive the security message if they are in one browser with multiple tabs.
If a user attempts to in log in using a different browser, with the same User ID without ending their existing session in their original browser, they will re-directed to the login screen with a security message in their original browser.
Users with this security feature turned off will not experience the occurrences described above when logged into different browsers or machines with the same User ID.