A Field Unit is a unit which causes unhappiness even when not aggressively deployed outside your nation's borders. In most rulesets, Field Units only cause unhappiness in Representative governments such as Republic and Democracy.
In most rulesets, Cruise Missiles, Bomber types, and Nuclear units are Field Units. (In Classic ruleset, Helicopters are also Field Units.)
In standard rulesets, a Field Unit causes 1 unhappy citizen in its home city under the Republic government, and causes 2 unhappy citizens in its home city under Democracy government.
Soft Field Units[]
Some server versions support the Peaceful_Field_Unit_Bonus effect., which modifies Field Unit effects. The Multiplayer II branch of rules used on Freeciv-web uses this effect. In that particular ruleset, the Medium Bomber is a Soft Field Unit.
Typically, Soft Field units cause 1 less unhappiness when peacefully deployed than when aggressively deployed. Thus, if they are not aggressively deployed, they do not cause unhappiness for a Republic. For a Democracy, they cause 1 unhappiness instead of 2.
Potential for other implementations[]
Negative values of the Peaceful_Field_Unit_Bonus make a city discontent to not have its units aggressively deployed. For example, it could be used to make cities unhappy to have certain "nasty" units present in their city (bio-weapons, mercenaries, monsters, and so forth.)