[Freesurfer] question(s) about brain volume calculation

I was wondering how FreeSurfer calculates brain volume. Is volume calculated from the voxels included in brainmask.mgz, from the area inside the generated Pial surface, or from summing the aseg volume statistics? Also, to exclude ventricular volume, do you just subtract the segmented ventricle label volume? I ask because I was wondering what freesurfer errors I should be concerned about most when looking at whole brain volume (pial surface including eye/skull, overzealous segmentation labeling, etc). Also, how should I consider whole brain volume regarding the difficulties freesurfer has calculating the surface of the hippocampus. Thanks for your help. -Nima