Alternate Class Features
Class Replacement Features are something that was pioneered by WotC as substitution levels, and adapted for Pathfinder RPG by Legendary Games — mini feature replacements akin to those found in archetypes, except that they allow you to pick and choose select features to replace those of a base class, and better personalise your character around a theme, without needing to multiclass or prestige class, and without the all-or-nothing approach of an archetype.
You do not have to select all replacement features listed in the given replacement class, nor do you have to take the feature at the level indicated, the stated level is only the minimum level you have to be in the class whose features you are replacing or the level at which you gain the feature you are replacing, if later. You may replace a feature you’ve already gained in your class progression; treat it as retraining said ability.