Most processes that the COHERENT kernel executes are associated with a controlling terminal. (The only exceptions are daemon processes that are started by the process init.) This terminal directs I/O to the physical device through which the user who invoked the process is accessing COHERENT. Usually, this is a serial port or the console, but it could also be a socket (in the case of a telnet or ftp session), or some other device.
The driver ct lets a program access the controlling terminal automatically. It is accessed through the device /dev/tty. Thus, when a program invokes the system calls open(), close(), ioctl(), read(), or write() on /dev/tty, driver ct directs those calls automatically to the appropriate driver for the controlling terminal. This spares applications from having to know the details of the controlling device -- all it has to do is manipulate /dev/tty and let ct take care of the details.