Special codes is a recent additions to give extra bonuses to those who have helped or supported the game in other forms.
Currently this includes Patrons and Fan Artists. The Patreon Link can be found on the game menu screen, and the Fan arts can be posted on the correct section on the bottom of the ITCHIO game page. Both have their unique code and item, which I either send personally or post.
I plan to expand these to more people over time, but its just a way to show my appreciation to those who have gone out their way to support the project in other ways. <3