Which International Astronomical Union rule does Pluto not meet for planet status?

In 2006, the International Astronomical Union (IAU) established three formal conditions for a celestial body to be classified as a planet. Pluto meets the first two requirements: it orbits the Sun and has sufficient mass for its own gravity to have molded it into a nearly round shape.1

However, Pluto does not meet the third requirement: it has not "cleared the neighbourhood around its orbit." This means that Pluto is not massive enough to have exerted the gravitational dominance necessary to remove other rocky and icy debris from its orbital vicinity. Instead, Pluto orbits partially within the Kuiper belt, a region populated by numerous other objects, and is considered part of that debris field. Because it fails this specific criterion, the IAU reclassified Pluto as a dwarf planet.1234