Stack leniency
Stack leniency is a parameter of a beatmap which controls the formation of automatic stacks of circles and sliders in the osu! game mode. A lower stack leniency reduces the maximum amount of time after which hit objects are no longer placed in the same stack; likewise, higher stack leniency will group together objects that are placed further in time.
Stack leniency may be changed from the Advanced tab of the Song Setup section of the beatmap editor. While its displayed possible values in the editor range from 0 to 10, they map to the 0.0–1.0 range of the StackLeniency parameter in .osu files.
Behaviour
Stacking hit objects while editing
Stacked hit objects while playing
Stack leniency, together with the approach rate, controls which circles and slider parts are considered for stacking.1 The stack is composed of the objects which are no longer than preempt * StackLeniency milliseconds apart, where preempt is the approach window width, and StackLeniency is the value taken from the beatmap's .osu file.
The minimal stack leniency value, 0, disables automatic stacks completely, while at its maximal value, 1, objects are considered stacked from the moment they start becoming visible.