sd_ant_channel_assign doesn't actually open the channel. See Figure 8-2 in the ANT Message Protocol and Usage document.
Correct you choose whatever radio channel number you want of the channels not yet in use on your device. See Figure 8-3 in the ANT Message Protocol and Usage document. Two devices are communicating over a channel to each other. One uses radio channel 0 on it's radio, and the other radio channel 1 on it's radio. Different radios have different numbers of channels available. The channel number is irrelevant other than allowing the different operations such as assign, set, open to apply to the channel. Simple systems typically always allocate a fixed radio channel number to a specific profile purpose. Complex systems allocate channels dynamically as profiles are enabled and disabled. The light channel will see each of the possible numbers at some point in the products life.
For bike lights the ANT+ profile specifies the network (ant+ key and RF frequency), channel type, and device type as constants. For the broadcast channel the device number and upper nibble of the transmission type byte are determined by you (typically from device serial number), and the lower nibble of the transmission type byte is a constant defined by the profile. The device number and transmission type (whole byte) are communicated in the connect command for the shared channel.
