I thought it was the volume of the element, but it seems to be a different thing. For example, in a C3D8 element the IVOL provided at the Centroid is 1/8 of the total volume and not the total volumen itself.

I also thought that IVOL at the Integration points would represent the volume of the part corresponding to the that Integration point (IP), but it does not appear to be so. For example, in a 1x1x1 cube, IVOL at the IPs are all equal to 1/8. However, if one shift only one edge, IVOL does not represent the volume of each eighth of the element anymore.