Yu Lee was making a mod for the game Terraria. Normally, when you play Terraria, there is a bar at the top left of the screen where you can add 10 different things from your inventory, so that when you click the buttons or press the corresponding keyboard keys, you equip that particular item.

In Yu Lee's mod she was adding a magic system to the game, so the first 6 buttons on the bar did magic spells instead of being assignable to inventory items. The first 3 spells that she added we different types of projectile spells that fired out blasts of energy with different strengths, speeds and ranged. The last 3 spells were utility spells that did things like letting you teleport a short distance or create a shield to defend against enemy fire.

I tested her version of the game, but I found it annoying that the first six slots were always assigned to the spells, so that even if I didn't have any power left for a specific spell it was still taking up a slot on the button bar. I told Yu Lee that it was really annoying only being able to equip 4 normal inventory items instead of the usual 10, and that I kept having to reassign things from the menu, but she was interested in my feedback and thought I was just trying to criticize her.