4.28.1 Selecting a serial line to connect to
The ‘Serial line to connect to’ box allows you to choose which serial line you want PuTTY to talk to, if your computer has more than one serial port.
On Windows, the first serial line is called COM1, and if there is a second it is called COM2, and so on.
This configuration setting is also visible on the Session panel, where it replaces the ‘Host Name’ box (see section 4.1.1) if the connection type is set to ‘Serial’.