Hello. Since I couldn't find this info anywhere else, here's how some of the Boosts work in this mod, from my in-game testing.

Weapon Physical Boosts
The ones just from the grindstone. This can be reapplied as many times as you want, and it recalculates the boost value for your current level. Level up? Take your weapons to the grindstone. This boost also works on weapons that don't deal Physical damage. It actually boosts the Fire on a Fire Sword, for instance. It's just a boost to the base damage, whatever that is. Using the Item Leveler will NOT scale this boost up to the new level, it must be reapplied.

Other Weapon Boosts
Things like +1 Strength, or +Bleeding. These can only be applied once each, reapplication does nothing. They all can be used with one another. Using the Item Leveler will keep these bonuses intact, as they are not level dependent.

Armor Physical Boosts
These work just like the grindstone buffs.

Other Armor Boosts
These work just like the Other Weapon Boosts

Jewelry Boosts
Filling Empty jewelry with gems means they aren't Empty anymore. Since only Empty jewelry can be boosted, this system doesn't work like the others. While Weapon and Armor Boosts are for improving gear that is already good, the Jewelry boosts are for making basic magic gear, and they can't be improved from there.