Goal: Calculate the sum of the volume, area, and length of scoped entities.

Code:

sum = 0

for geoid in  ExtAPI.SelectionManager.CurrentSelection.Ids  :
    geoEntity = DataModel.GeoData.GeoEntityById(geoid)
    if geoEntity.Type == GeoCellTypeEnum.GeoBody:
        sum += geoEntity.Volume
        type = "volume"
    if geoEntity.Type == GeoCellTypeEnum.GeoFace:
        sum += geoEntity.Area
        type = "area"
    if geoEntity.Type == GeoCellTypeEnum.GeoEdge:
        sum += geoEntity.Length
        type = "length"

# values are reported in the CAD unit system so get that    
unit = Model.Geometry.LengthUnit
print("Total selected "+ type + " is: " + str(sum) + " "+ str(unit))