[Pluto]

The signet ring you can set to display a seal of some sort to represent your character. For example, Gust has his as a two-handed sword in the foreground with a millhouse at the end of a winding road in the background. The wax is for the color you want the seal to be.

So, type up your letter and then equip your signet ring (with wax in your inventory)and use it on your letter. You now have a letter stamped with your seal.

I've only done this with sealed letters. I guess it works with other pieces of parchment? Not sure....