Item categories are used to determine what the item does when used by the player. You may want to add new item categories if you want to add new functionality for when an item type is used.
To start go to the E_ItemCategorys then you can select the New button to create a new category and name it. You have now created a new item category that you can set in the item list like shown in the image below.
Next if you want to add new functionality to your new category you can go to the BP_PlayerInventoryComponent and then to the Use Item function. Then find the Switch node shown in the image below there are also comments giving you some suggestion for nodes.
Once you have the code done when you use the item either though the right click menu or using the quick slot key your new code will be run.