The Portal Login Security Options are:

1.  This is a public or shared computer

Select this option if you are connecting from a public computer. Be sure to log off and close all browser windows to end your session. 

Please also refer to http://www.microsoft.com/protect/yourself/mobile/publicpc.mspx for more information regarding the security risks associated with using a public computer.

or

2.  This is a private computer

Select this option if you are the only person using this computer. This option provides additional time of inactivity before automatically logging you off.