The cobalt percentage in drill bits indicates the cobalt content in the high-speed steel (HSS) used. 5% Cobalt Bits: These contain around 5% cobalt, offering decent durability for general drilling in materials like steel, aluminum, and wood. They might not handle extreme heat as well as higher cobalt content bits. 8% Cobalt Bits: With roughly 8% cobalt, these bits are more heat-resistant and durable. They're suitable for harder materials like stainless steel and cast iron, handling higher drilling temperatures effectively. Higher cobalt content generally improves heat resistance and durability, especially for challenging drilling tasks.