Company uses negotiated fares to get you from your assigned domicile to whatever city they need you in. Same in reverse back to your domicile. When a commuter elects to "Deviate" from his pairing and fly from his/her hometown they can. If they can buy a ticket to get to work for less than company fare your golden. If it costs more for you to get there, that's fine, you just owe company the difference. Look it's pretty involved but that's it in a nutshell