(a) The classes of actions in this section are those that the Postal Service has determined do not individually or cumulatively have a significant impact on the human environment. To be categorically excluded, it must be determined that a proposed action fits within a class listed and there are no extraordinary circumstances that may affect the significance of the proposal. The action must not be connected to other actions with potentially significant impacts or is not related to other proposed actions with potentially significant impacts. Extraordinary circumstances are those unique situations presented by specific proposals, such as scientific controversy about the environmental impacts of the proposal, uncertain effects or effects involving unique or unknown risks.