Bonus Multiplier Slots
A bonus multiplier applies inside a feature round only, and its schedule, targets and persistence are stated in the rules of that round.
How bonus multipliers work
A bonus multiplier is a factor that exists inside a feature round and nowhere else. The base game pays at face value, and entry into the round brings the factor into play. The rules of the round state where the factor comes from, what it applies to and whether it survives from spin to spin.
The common sources are fixed schedules, symbols that land during the round and progress markers. A round can open at a stated factor and climb, attach factors to individual symbols, or apply one factor to the round total when it closes. Each design reads differently in the rules, and the wording decides how the arithmetic runs.
The defining questions are persistence and target. A factor that persists across the spins of a round compounds the whole feature, whilst a factor that resets each spin only scales single results. The target matters just as much, because a factor on one win, on one spin and on the round total are three different features wearing one name.
Bonus multipliers usually appear inside Free Spins, and the growing variant has its own page under Increasing Multipliers. The general behaviour of factors, with the rules for combining two of them, is covered under Multipliers.
When a round is advertised on its multiplier, the advertised number is the top of the schedule and not its typical value. The rules give the schedule; the schedule, read in full, is the honest description of the feature.