My understanding has been that building volume for ACH50 is generally calculated as the volume where air is actually exchanged so basically floor to ceiling to walls of all conditioned space.

RESNET appears to specify a much larger volume and isthe total volume to the exterior face of the building (The sum of the Conditioned Space Volume and Unconditioned Space Volume) minus the volume of: Floor cavities that have Unconditioned Space Volume both above and below, Unconditioned wall cavities, Vented crawlspaces, Garages, Basements (if the door between the basement and Conditioned Space Volume is closed during enclosure air leakage testing (Section 3.2.5), and, Thermally isolated sunrooms.