When you enter a usual driver to a rental car, that person will receive a SMS as a reminder when the replacement car is reserved or coming back from a reservation.
To set this, go to System - Options and search:
'SEND_USUAL_DRIVER_START_REMINDER_SMS (at the start of a reservation) or SEND_USUAL_DRIVER_STOP_REMINDER_SMS (at the end of a reservation).
If this configuration is set to "always" , the usual driver will automatically get an SMS .
You will be sent a standard SMS. You can adapt this text in the screen Administration - Comm. Templates.
Before, an email was also sent to the usual driver at the start / cancellation / modification of the reservation. You can see this email in the communication screen in the block of the reservation. You also can change this text in the screen Administration - Comm. Templates.