What is the "Channel" parameter of the NVR software

Channel is a NVR channel configuration parameter (as illustrated by the image below), which identifies the video source of a potentially multi-channel device. Normally the value is "1" and it means the first video source. If you change it to "2", it will select the second video source, and etc.

In the terminology of ONVIF, this mostly means that among the video sources returned by GetVideSources, only that matches what is specified by Channel will be used, while among profiles returned by GetProfiles, only the profiles matching the aforementioned video source will be used.