Integration Method:
The first method, Integration Method, will calculate volumes based on 3D integration. This volume is an approximation, as it is generated by piercing vectors through the solid and totaling the lengths between where they enter and leave the solid, then turning that into a volume, based on the spacing between the piercing vectors. The closer the vectors, the more accurate the result, but the slower the calculation. The spacing of these vectors can be set in the Project Settings | Volumes tab. Set the Default Subcell Count for Volume Calculations to a higher number to give a more accurate approximation.
When calculating the volume using the Integration Method, a confirmation message is displayed when the solid has openings &/or self-intersections. The checks are only performed if the options, "Check solids for openings" and "Check solids for self-intersecting faces" are enabled in the Project Settings. A check for duplicates is not performed, as volume does not depend on duplicate faces.
Note about Invalid vectors: These can occur when the Calculate Volume Tool or the Generate Partials Tool cannot determine which segments of a given vector are inside the solid due to openings in the solid or when the vector skims a face of a solid.
Analytical Method:
The second method, Analytical Method, will calculate the true mathematical volume of the solid based on a 3D matrix determinant calculation. This method is 100% accurate and automatically checks if any selected element or any solid inside of a merged shell element has openings. If openings are found, the volume calculation is terminated and a warning is issued to the Message Window. Analytical Volume can be performed either through the Calculate Volume dialog or more directly through the Calculate Analytical Volume(s) with Selection function. Analytical method requires perfectly closed solids. Integration method will skip holes, so if there are tiny sliver openings (which can happen after an intersection operation), the integration method can still be used, and get an accurate result. Only if the holes are large, will the Integration method under-report the volume.
To compute analytical volume, a solid should completely bound a volume. If a surface does not completely bound the volume, most likely it has openings. However there may be a rare chance for the surface without openings to still not separate space into "inside" and "outside". Such a surface is called one-sided or non-orientable. A simple example is a Möbius strip. A volume cannot be computed from a non-orientable surface.
In regards to both methods:
When performed through the Calculate Volume dialog, both methods will only calculate the volume of a single element. If the volume of multiple solids must be calculated, they must either first be merged into a single element, or the volume must be calculated using the Calculate Analytical Volume(s) with Selection function, which will allow multiple solids to be included and totaled in a volume calculation.
To calculate a volume of a solid with a void totally internal to it, use the Integration method. If the void intersects the outer solid, then use the Intersector Tool first to build a correct solid, with the void 'cut out' of it. Also make a copy of the void solid, place it in the same object as the main solid, and merge the void solid and the main solid into one object. This way, both the main solid and the void as one solid can be used to calculate the volume. Caution: If using this same merged solid (main and void) and computing an Analytical volume, a combined volume will be calculated and reported, which would be incorrect.