Extension 0 is typically used to represent the "Operator" in an auto-attendant.

Extension 8 is reserved for dial-by-name directory.  

Extensions 1-7, and 9 are Department Extensions (Example: Sales Ext 1, Support Ext 2, etc.)

Extensions 700-799 are Employee Extensions (Example: John Ext 702, Tom Ext 703, etc.) 

Extensions can be registered to an app, or can forward onto (one or more) forwarding numbers. The forwarding numbers are the number of the actual device which receives the phone call such as your cell phone. Each extension has a voicemail box and the ability to control a variety of settings including the call forwarding schedule, ring rules, and caller ID display.