Both cu and ct belong to Intermachine communication commands. Intermachine communication commands are one which is used for communicating between two machines.cu command is used to dial a phone number and there by attempts to make an interactive communication between two machines.ct command dials the phone number of a modem which is connected to a system and thus creates a login process and thereby attempts to make an interactive communication between two machines.So both cu and ct belong to Intermachine communication commands.