This counts uncorrectable Header Check Sequence errors on this port since the last reset, cell headers so corrupted the HEC algorithm could not fix them, meaning those cells were dropped. Because these are cells actually lost rather than merely corrected, a nonzero and climbing count here on this specific port represents real, measurable cell loss impacting whatever traffic is carried on it, unlike the merely-informational pportHcsErrors. A port showing hundreds of pportHcsSevereErrors per hour is actively losing that many cells' worth of user data per hour and needs the physical link or optics investigated rather than left to self-correct.