[alsa-devel] Selecting correct input device port.

As I learn't in this mailng list, a device can typically handle only one stream at a time unless it supports hardware mixing which's exposed via subdevices in ALSA

What does subdevice mean for input device? Do they mark physical ports for input? If not how do I record from a specific input port? Is it automatically selected by the card?

And what's 'input source' in ALSA mixer?