| 'Gross' volume is the internal volume of the enclosure before the port volume is subtracted. If you're using external dimensions and you set the wood thickness to something other than '0', the wood thickness will be subtracted. |
'Net' volume is the internal volume after the port volume is subtracted from the gross volume. If your speaker needs an enclosure that's 1.5 cubic feet ported at 35Hz, the 'net' volume must be 1.5 cubic feet for proper operation.