From what I know [hostname or ip]:[display number] is just another way to specify the destination port. Or in case of Linux VNC server to specify Linux display, which is not the same as physical monitor.
So for exmaple if you enter IPADDRESS:1 is the same as if you connect to IPADDRESS on port 5900
, in case of IPADDRESS:2 it will be equal to IPADDRESS on port 5901
Kindest Regards,SmartCode Solutions Support