How does ITK-SNAP calculate volume of segmentation?

[Daniel]

I am wondering how ITK-SNAP calculates volume of segmentatons.

I noticed one possible way is: (1) use (0028|0030 pixel spacing) to get "length" and "width" of the voxel (2) use [(0018,0088) Spacing Between Slices] to get the "height" of the voxel.

I tried to multiply the three values and the resulting Volume was nearly the same as the one given by ITK-SNAP. 

My first question is "does ITK-SNAP calculate volume this way?".

Besides, I just read a few discussion today (for example, https://stackoverflow.com/questions/14930222/how-to-calculate-space-between-dicom-slices-for-mpr), which say the parameter "Spacing Between Slices" is not always reliable for such tasks. But I dont quite understand the conclusion and their discussion.

My second question is "Can I use this parameter to calculate the 'height' of the voxel?"

Thanks in advance!