"The government's ability to restrict speech on public property depends on the type of forum where the speech is occurring. Public forums, spaces like public parks and sidewalks that have long been held open for speech activity, receive the highest degree of speech protection. Designated public forums, spaces that the government has affirmatively decided to open to speech activity, also receive the highest degree of protection. Limited public forums are spaces that the government has opened only for speech on particular topics, or at particular times. Speech restrictions within limited public forums are permissible so long as they are reasonable and viewpoint neutral, a less strict standard than that applied to public and designated public forums. Nonpublic forums are forums that are not held open for speech activity, though they may be open to the public for other reasons. The government has wide latitude to restrict speech in nonpublic forums in order to preserve the property for its intended purpose. Speech restrictions in nonpublic forums are permissible so long as they are reasonable and viewpoint neutral. The interior of the Supreme Court Building is a nonpublic forum, as the protesters acknowledge in their motion to dismiss. Consequently, the ban on making harangues and engaging in loud speech (which are two separate offenses) need only be reasonable and viewpoint neutral to be upheld."
CRS Legal Sidebar, October 6, 2015
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html