In Arma, the following objects are on side civilian: dead bodies and vehicles, empty vehicles, all mission editor placed objects (that do not have an inherit side), all objects with interactive components such as ladders and doors, man made structures such as buildings (including classless wrp placed objects), docks, high tension powerlines, see-saws, large rubbish bins, fountains. Basically if an object uses a non-simple damage or physics simulation it is likely to be on the civilian side.
NOTE: If you need to know the side of a dead body, you can use faction command as a workaround -- Killzone_Kid
Ceeeb
In Operation Flashpoint v1.96, the side return value for a vehicle is based on the side of its commander, then gunner, then driver, then cargo. It will retain its side value until it is either empty, or a unit of another side takes over in a equal or higher role, irrespective of the side of other units still on board. This can be used to simulate friendly fire, as a vehicle can be made to appear to be an enemy even though all units on board are actually friendly.
Tankbuster
As Ceeeb says above, in Arma 2, side for a vehicle often depends on who the command or driver is, for example, A KA52, piloted by a USMC guy will have side WEST. For an accurate result of what 'where the vehicle was made', use faction. This ignores the pilot/commander, so in my example here, this KA52 will always return faction "RU". Note that faction returns different values to side though.
RKurtzDmitriyev
Side values for ambient life (animals) are bizarre. side _unit returns "CIV", but playerSide returns "AMBIENT LIFE" when the player is an animal. In that case, playerSide == side player; returns false!
Fortunately, you can easily check if a unit is an animal with _unit isKindOf "ANIMAL";