Hi
My interpretatation is that the total % of CO2 plus O2 must lie between 0.5 - 6%. So, if you have say 2% O2 and zero % CO2, then the total of CO2 plus O2 is 2 + 0 = 2%, which lies between 0.5 and 6%, so meets the requirement. I don't think the intention is necessarily for a tri-mix, but does accommodate a tri-mix.
Regards
Niekie