You open one of a major. Partner raises you to two, showing support with 6-10 high card points. You are a rebidder with an intermediate hand, 15-17 high card points. Opposite an average of "eight" points, you have enough to try for game, but not enough to insist on it as you would with an 18-count.
So you rebid three of a lower ranking suit (usually a minor, but even hearts if your suit is spades) as a "help suit" try to see if there is "something extra" to propel you to game.
Here's where I am confused. Most players in my club use what I call a "short" help suit convention. That is, a bid of a new suit at the three level shows a singleton or void. The idea is to identify (cross) ruffing values that will allow a game to be made with 22-23 high card points.
In the version I was taught, a three level bid in a new suit shows side suit controls, typically a king or ace, with 3-4 cards in the suit, and a potential for a double fit and several side suit tricks. Let's say a partnership has a double fit of two eight card suits and 17-18 high card points. That leaves ten cards in two more suits, hopefully divided 2-3 opposite 3-2, that is, four losers counting ruffing values. But there will be an ace or king left over (22 minus 18) to reduce the losers to three.
There's even a third version in use at my club, where the three level side suit bid shows controls: either a singleton or void, an ace, or a KQ combo.
Any system is subject to partnership agreement. But which (or all) of these systems is in general use and how do they work? In any case, have I confused any alleged system with something else?