Hi there,
Yes, most float attribute computes an average/luminance on input channels.
The reorder doesn't do anything else than copying/changing the channel value of the input.
If you have R G B A to respective values of 0.1 0.2 0.3 0.4 then
rrrr will output 0.1 0.1 0.1 0.1
gggg 0.2 0.2 0.2 0.2
bbbb 0.3 0.3 0.3 0.3
aaaa 0.4 0.4 0.4 0.4
This is explained here: https://www.clarissewiki.com/4.0/reorder.html