In case the dominating spectrum in the room is known, this spectrum may be used as a reference spectrum as well.
Click on the file selector symbol to enter the spectrum, insert a new row via the table's context menu commands Insert before/after, and enter the linear (unweighted) reference spectrum.
In any case, CadnaR calculates from those levels a normalized reference spectrum. This enables to calculate the sound decay curve - referring to the total level in dB(A) - for a source with an arbitrary spectrum.