The Pro Destroyer is an essential disc for any serious distance thrower. Factory Second discs are discounted because they have minor blemishes and/or multiple stamps. They perform the same as stock discs.
More about Factory Seconds:
A Factory Second disc has cosmetic imperfections which do not affect flight or durability. These discs may contain bubbles or flecks in the plastic, other cosmetic imperfections, and/or multiple stamps. Stamp size, design, color, placement, and number of stamps on each factory second disc may vary widely.