The "Pickup Sets" basically allow you to define specific Item Sets (like the ones defined in your character's Item Set Manager) to be added when the character interacts with that ItemPickup. But you can also just use the "Item Definition Amounts" to simply add specific item definitions to the player's inventory, when you don't need to define an entire new Item Set (e.g. in the demo scene, there are already Item Sets defined for the Sword, Assault Rifle, etc).