Thats the quality of the item, it determines how well an affix will roll when you add a new one, or apply flourite and i think diamond?
I’m unsure on diamond as it almost always rolls terrible anyway…
Quality also affects a weapon’s base damage and armour’s base armour rating.
Max quality is 25, but eternal items have 50.
You can increase quality using emeralds
it affects the chance of getting better/higher/max affix values when using Diamond (rerolls all Crystal, Legend, Epic, and Normal affixes on one Item), Fluorite (rerolls one Epic or Normal affix of your choice on one Item), and Obsidian (rerolls the Crystal affix on one Item. must have one empty space on the Item. if you don’t have a Crystal Affix on an Item, you will be given 3-6 affixes to choose from).
when rerolling affixes on Legend Items, affix values don’t go below 75% of an affixes maximum value.
when rerolling affixes on non Legend Items, affix values will range from the affixes minimum to maximum value. a higher Item Quality means a better chance for middle to higher values.