[Freesurfer] A question regarding volume calculation

Dear experts

I'm currently preparing a paper using the results of automatic volumetric segmentation of subcortical and cortical grey matter structures from aseg.stats and aparc.stats files.I have cited previous article by Desikan RS, et al (Neuroimage 2006;31:968-980) for the cortical GM volumetric segmentation, and Fischl B, et al (Neuron 2002;33:341-355) for the subcortical GM volumetric segmentation.At this point, my question is how volumes of these structures calculated. Is this volume of each GM structure based on surface area multiplied by cortical thickness? Apology for the newbie's question.Thank you in advance for your help. Kim MJ