What is a channel (on a port) - not an RF channel

I was reading thru the documentation for /too sms - and it speaks of a channel. I’ve been trying to figure out what a channel is exactly. My wildest guess is there is some weird way to use the serial port with multiple devices, but I have no idea how that’s implemented. My instinct says I’m wrong. Does anyone who what this is all about? I’ve love to hook up both a GSM and a GPS unit to my RouterBoard 1000 at the same time, but there is only one serial port.

thank you

If it were RS485, it could do that. Dunno if MT supports it, but the UART in RBs is only RS232.