On the CTI+ tab, you define the link to a number, with which the user can control an external phone. With the option "Also deliver calls to this number when SwyxIt! is not running or CTI is switched off" you ensure that incoming calls are forwarded to the external phone independently from SwyxIt!, even if the computer of the user is shut down or CTI is deactivated.
You need an appropriate license for the use of SwyxCTI+. See Configure a CTI pairing to the number of an external phone.