Descriptors meaning Jarvis DFT

[giotre]

When featurizing a Crystallographic Information File with CFID, descriptors like "C-m" are listed as "Elastic constants of an element from JARVIS-DFT (total 36)". What is the meaning of that m-th constant? What does mean "elastic constant" of an element (atom?)? And why are they 36?

[knc6]

These are elastic constants of an element's ground state solid phase, so for example, FCC Aluminum's 6x6 elastic tensors, which are flattened into 6x6 = 36 dimension array. Hope this helps.

[giotre]

Thank you, now it's clear.
I have only another question: what is the meaning of vpa, "Volume per atom of the cell"? Taken a cell, is it the average volume of an atom? And, if so, what would be the difference with vol_pa?