Sectional Volume Methods as described in the help.
The Average End Area is probably the most common and also the least accurate.
It's also what many agencies specify as the method to be used on their projects for computing volumes.
The accuracy all of the 3 volume methods are dependent on many variables such as how accurate the survey is
the surface or surfaces were created from etc.
The Prismoidal & Composite are more accurate because they take into account triangle information in between sample lines.
If a site is very flat and sample lines are placed appropriately their likely wouldn't be a lot of difference between the three methods.
With a lot of terrain on a site, the volume method differences will be more noticeable.
C3D 2018.1
C3D 2016 SP4
Win 7 Professional 64 Bit