from the Wow API - http://www.wowwiki.com/API_SetLootThreshold

/script SetLootThreshold(2)
  • 0 - Poor
  • 1 - Common
  • 2 - Uncommon
  • 3 - Rare
  • 4 - Epic
  • 5 - Legendary
  • 6 - Artifact

Settings below 2 or above 6 will cause errors.

mobs holding loot below the threshold will not be lootable. Mobs holding loot at or above the threshold will sparkle