Speed is the whole distinction. The Moon changes sign every two and a half days, the Sun every month, Mercury and Venus every few weeks, Mars every six or seven. These are personal: two people born in the same month can differ on all of them. Jupiter takes a year per sign and Saturn two and a half, so they describe a class year rather than an individual — social planets, in the usual phrase.
Uranus spends seven years in a sign, Neptune fourteen, Pluto anywhere from twelve to thirty-two. Everybody in your school year shares them. When a reader tells you your Pluto in Scorpio makes you intense, they are describing several hundred million people at once.
The exception matters more than the rule: when an outer planet sits close to a personal point — conjunct your Sun, Moon or Ascendant — the generational placement lands personally after all. That is where the genuinely unusual charts come from.