A more thorough explanation:

Definition: Prop. reg is an abbreviation for proposed regulation, which refers to a draft administrative regulation that is circulated among interested parties for comment. Regulation, on the other hand, is the act or process of controlling by rule or restriction. It can also refer to a legal order or rule issued by an administrative agency.

• The Environmental Protection Agency (EPA) issued a proposed regulation to limit carbon emissions from power plants.
• The Federal Aviation Administration (FAA) issued a regulation requiring all commercial pilots to have a certain amount of flight experience.

These examples illustrate how regulations can be used to control certain industries or activities. The prop. reg example shows how agencies can seek input from interested parties before finalizing a regulation.