It's referring to "Irregular Warfare" perhaps?
test
Moses and Elijah on the Mt. of transfiguration represented the Law (Moses),
and the Prophets (Elijah).
The purpose for this, their standing beside Him,
was to show Jesus Christ was fulfilling all the Law and fulfilling all that the Prophets foretold about Him.
The theory it's Elijah physically returning because he had been taken into heaven,
isn't consistent with the fact Moses hadn't been taken into heaven.
Elijah no more returned than did Moses.