what is purpose of "$(O)" in top-level docs Makefile?

  almost certainly a dumb question but was perusing the top-level docs
Makefile and read at the bottom:


# Catch-all target: route all unknown targets to Sphinx using the new
# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
%:
        $(SOURCEDIR)/set_versions.py
        @$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)

  it's unclear to me what the point of $(O) is if it's supposed to be
a "shortcut" for $(SPHINXOPTS) when they're both incorporated in that
build command. can someone clarify what is going on here?

rday