…used to support a building (Judges 16:26, 29)

…as a trophy or memorial (Genesis 28:18; 35:20; Exodus 24:4; 1 Samuel 15:12, King James Version, “place,” more correctly “monument,” or “trophy of victory,” as in 2 Samuel 18:18)

…of fire, by which the Divine Presence was manifested (Exodus 13:2)

The “plain of the pillar” in Judges 9:6 ought to be, as in the Revised King James Version, the “oak of the pillar”, i.e., of the monument or stone set up by Joshua (24:26).