What are refhg, refmark, refhgb?

This question has this code block:

setlength{textwidth}{6.5in}
setlength{textheight}{8.5in}
defrefhg{hangindent=20pthangafter=1}
defrefmark{parvskip 2mmnoindentrefhg}
defrefhg{hangindent=20pthangafter=1}
defrefmark{parvskip 2mmnoindentrefhg}
defrefhg{hangindent=20pthangafter=1}    %20pt
defrefhgb{hangindent=10pthangafter=1}
defrefmark{parvskip 2mmnoindentrefhg}

What are refhg, refmark, and refhgb doing in there, and why are they repeated?

I’ve inherited a doc with a block almost exactly like this, down to the %20pt comment, so it’s probably copied from somewhere. Web searches for "refhg tex" are not fruitful.