The AVG provides a simpler means
of automatically convolving with a triangular line shape, which is the
recommended option if all you need
is an output grid which represents the average values over spectral intervals
rather than discrete samples of the spectrum.
The incompatibility with the OPT flag
is on the basis that instruments do not measure optical thickness directly,
so spectral averaging in 'optical thickness' is meaningless. If you really
do want this (and are you sure you do?) output the convolved transmittance
spectrum τ and note that optical thickness χ = - ln τ.