Rocket League ranks every competitive player on a ladder of eight tiers: Bronze, Silver, Gold, Platinum, Diamond, Champion, Grand Champion and, at the very top, Supersonic Legend. Every tier below Supersonic Legend is split into three ranks, from I to III, so the ladder climbs from Bronze I all the way to Grand Champion III. A rank boost takes your account from the rank you hold today to the target rank you choose, and the order is finished only when that target is reached.
Each of those ranks is further divided into four divisions, numbered I to IV, which show how far you are through the rank. Winning pushes you towards the next division and eventually the next rank, while losing pulls you back and can drop you a rank if you slip under its lower edge. Because there are four divisions inside every rank, even a single rank of progress usually means a long run of games. A booster measures the order against these same divisions, so you can see exactly where the climb stands.
The ladder is not shared between modes. Duel is played one against one, Doubles two against two and Standard three against three, and each of these competitive playlists keeps its own rating and its own rank. Extra Modes such as Hoops, Rumble, Dropshot and Snow Day have separate ranks as well. When you order a rank boost you therefore pick the playlist it applies to, and the booster only plays in that queue.
Supersonic Legend is the peak of the ladder and has no ranks or divisions above it. Only a small share of the player base ever reaches Grand Champion or Supersonic Legend, which is why these targets are priced very differently from a climb out of Gold or Platinum. Season rewards are earned by winning matches at your rank, ten wins for each reward level, so a higher rank is not only a badge in the lobby but also the way to the better rewards at the end of the season.