1: A tag that prevents weapons from appearing in loot drops like research stations all together. Even if setting a weapon's tier to 5 prevents this (and I'm not sure it does) it also prevents the weapon from being sold in markets.
2: A hint for weapons that tells the AI to ignore a weapon's ammo completely. Weapons that try to use Ammo-Regen setups as a standard burst mechanic (there are several reasons why this might be desirable in certain cases) are hampered by the fact that even with a DO_NOT_CONSERVE hint, the AI is still somewhat stingy with the weapon's ammo, and thus cuts the weapon's effective DPS harshly.