Current closed shape Sketch To Scale tools generate area as their "primary" quantity. While a depth value can be entered in the measurements panel the volume value is only displayed in the "volume" column - the primary quantity is still an area.
Having the volume available in the markups list works well if you are exporting the data for further processing outside of Revu, but it would be handy to have the option to have the volume as the primary quantity when working strictly in Revu.