asterisk/doc/extensions.txt

Extensions are routed according to priority, and may be based on any set
of digits, #, and *.  For each extension, several actions may be listed
and each given a priority.  When each action completes, it generally moves
to the next priority (except for some modules which use explicitly GOTO's. 
Extensions frequently have data they pass to the executing application
(most frequently a string).  There are some reserved application names: 

	GoTo:	Go to a specific extension
	Answer:	Answer the line, if it hasn't already been answered
	
Also, there are some extensions with important meanings:

	s:	What to do when an extension context is entered (unless
		overridden by the low level channel interface)
	i:	What to do if an invalid extension is entered
	t: 	What to do if nothing is entered in the requisite amount
		of time.

And finally, the extension context "default" is used when either a) an 
extension context is deleted while an extension is in use, or b) a specific
starting extension handler has not been defined (unless overridden by the
low level channel interface).