These are error handling extensions....
so for example, lets say you have some sort of IVR programmed...and a user is supposed to input a number or option on their keypad... "i" would be used for invalid input...whether it's an invalid extension or invalid option...and you would use this to either loop back to the begining of the menu, or do something else... "t" would be the timeout...where they waited to long to input something...and you program it to do whatever at that point...
You don't HAVE to have them, but they are useful when you want to make a stupid proof IVR menu...or you want asterisk to do something other than just hangup when a user dials an invalid extension or waits too long during an IVR menu..
http://www.voip-info.org/wiki/index.php?page=Asterisk+config+extensions.confhttp://www.voip-info.org/wiki/view/Asterisk+i+extension
