setChannel

(Usually) If set, this indicates the index in the secondary_channels table that this packet was sent/received on.
If unset, packet was on the primary channel.
A particular node might know only a subset of channels in use on the mesh.
Therefore channel_index is inherently a local concept and meaningless to send between nodes.
Very briefly, while sending and receiving deep inside the device Router code, this field instead
contains the 'channel hash' instead of the index.
This 'trick' is only used while the payload_variant is an 'encrypted'.