By spending Experience points characters can increase their Stat or Skill, improve Vitality and Health, or acquire additional Actions.
Every upgrade category is tracked separately.
Every upgrade provides +1 to stat.
Cost of upgrade is equal to absolute value of stat mod, but no less than 1. To upgrade from -3 to -2 character will need to spend 2 points. To upgrade from +4 to +5 character will need to spend 5 points. Upgrade from -1 to 0, and from 0 to +1 will cost 1 point every upgrade.
Every upgrade provides +1 to skill, or allows proficient rolls.
Characters can upgrade only 1 level of skill at a time, and price for upgrade is equal to "2 * target level". This way upgrade from 0 skill to 1 skill will cost 2 points, and from 2 skill to 3 skill will cost 6 points.
It is also possible to get proficient in skill, which is possible only at level 2 or higher, and cost 10 points.
Every upgrade provides +1d3 to max vitality, or +1d3 to max health.
First upgrade will cost character 1 experience point, second one will cost 2 more, third one will cost 3 more, and so on.
Every upgrade provides 1 additional non-universal minor action in selected category.
Cost of such an upgrade is equal to "2 * new amount of minor actions", so to get 6th minor action character will need to spend 12 points.