It has the "Wealth" ability, which means that you'll gain more Gildas when you kill a enemy with this brassard. Its in-game description reads "Sturdy armband with strong magic." You can find the Platinum Brassard in dungeons' chests, or you can buy it at G-Weapons in Gundorada Workshop for 900 Gildas when you get to the Chapter 5. It can also be bought at the Starlight Weapons for the same price and it's available when you get to the Chapter 5. This brassard is a medium-tier weapon (Tier 5) : it'll gain 4 Synthesis Points per level-up.