The "Limit Features for Extension" link, located under the Extensions tab, is used to disable the default PBX features for individual extension users.  You also use this tool to enable certain features that are not on by default so that designated users have additional capabilities.

Clicking on the "Limit Features for Extension" link brings up a listing of every extension that exists in the PBX dialplan.  Click on any extension name to bring up the modification screen.  The top portion of this tool lists all of the PBX-based features that are normally available to all extension users by default.  Activate the checkbox to turn off any of these features for a particular user.

The lower portion of this table lists the restricted-access features that are disabled by default for all users.  You will need to activate the appropriate features for a user if you want to grant him/her access to it.  This includes global Listen/Whisper/Barge, the ability to control the manual Night Attendant, and the ability to change messaging for auto attendants that are configured with dynamic message recordings.

The phone dialpad keypress codes that activate or access these features are listed on the "Modify Feature Codes" page, under the Additional Tasks menu tab.  A guide to using common feature codes is available HERE