max_damage allows to set the maximum durability of an item. Already existing items should keep their durability. Vanilla Mending and repairing with an anvil or a grindstone works, but not merging 2 items in a crafting table.
item_name set the default name of the item, so custom ones can be renamed without losing their default name when resetting it.
Two changes added with 1.20.5/6:
max_damage
allows to set the maximum durability of an item. Already existing items should keep their durability. Vanilla Mending and repairing with an anvil or a grindstone works, but not merging 2 items in a crafting table.item_name
set the default name of the item, so custom ones can be renamed without losing their default name when resetting it.