The default Visibility setting is Float. The visibility always reverts to float each time you enter a new world. Float means that the browser attempts to balance the current visible range with the desired frame rate you have set in the Performance Settings. When the visibility is set to float, the visibility will be farther in sparsely built worlds and closer in densely built worlds.
When a particular visible range is chosen from this menu, the browser will "lock" the current visible range at that distance, regardless of the current frame rate. Note that choosing high visible ranges in densely built worlds can result in extremely poor performance!
Note that distances in this menu that are less than the current world's minimum visibility will be disabled.