Each terrain type has a danger percentage, the chance to encounter a monster when stepping into it. Stealth fields change this percentage. Set them to Quiet for travelling, or Noisy when hunting for salvage. If you assume the zone thieves are a threat, EnviroBlurs can only be used in zone 6, UltraCloaks never. Still, if you're planning a long trip it's useful to know how many stealth fields you can make use of.