On level up, 100 stat points are obtained, and they may be spent on any of the stats that are rolls. Adding 1 to the [X] value of a stat requires 5*[X] stat points, adding 1 to the [Y] value of a stat requires 2 stat points, and adding 1 to the [Z] value of a stat requires 4 stat points. Once the character is done spending stat points (they may save any amount of them,) their new HPA is rolled, and that value is added to their HP. There is no limit to how many stat points you may spend in one stat, but bear in mind that putting too much into one stat makes others considerably less effective and unbalances your character.