I was looking at a couple of the metadata files describing the regions of GRCh38 (e.g. centromeres, alternate scaffolds, etc.), and one of the lines is the following:
HET7 7 61377789 61528020 HET na na na
This is the only "HET" region in the human genome, and I can't find any details documenting this anywhere in Google, I checked the Genome Browser and there isn't any special indication about the region, and I asked some knowledgable colleagues and they didn't know either.