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.
A rank statistic represents a ranking or level.
A growth statistic represents a place to accumulate something endlessly.
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_".