Statistic categories are used to qualify how a statistic works.
A core statistic represents a core or inherent value. It is primarily used for calculations.
Example: Strength, Agility, Wisdom, Spirit.
A rank statistic represents a ranking or level.
Example: Level, Class Level.
A growth statistic represents a place to accumulate something endlessly.
Example: Experience, Skill Points.
A store statistic represents a place to store a value, against a maximum. Every "store" statistic will generate a "Maximum" version of itself, with the formula name being prepended with "max_".
Example: Hit Points, Stamina, Ability Points.
Example: If a user defines a statistic named Hit Points, with a formula name hp, a "maximum" will be generated automatically called Maximum Hit Points with a formula name of max_hp.