Operational Environments describe what sort of environment or terrain a building or piece of equipment can be used. Some equipment is designed to be used on land or under the ocean's surface while other vehicles are more flexible. Some buildings can only be built in certain conditions (usually these are fairly obvious from the nature of the building or equipment) and you will be told if you are trying to build something in the wrong place. Finally, some buildings may only be built in an existing city or military base.