on the number of other countries its citizens are free to enter without visa. Most countries in the world require visas or some other form of entrance permit for non-citizens to enter their territory. Those who enter countries in defiance of regulations requiring such documentation are often subject to imprisonment or deportation. Exit restrictions in certain countries Most countries require that their citizens leave the country on a valid passport, travel document issued by an international organization or, in some cases, identification document. Conditions of issuance and the governments' authority to deny issuance of a passport vary from country to country. Under certain circumstances, countries may issue travel documents (such as laissez-passer) to aliens, that is, to persons other than their own citizens. Having a passport issued does not guarantee the right to exit the country. A person may be prohibited to exit a country on a number of reasons, such as being under investigation as a suspect, serving a criminal sentence, being a debtor in default, or posing a threat to national security. This applies to aliens as well. In some countries prohibition to leave may take the form of revocation of a previously issued passport. For example, the United States of America may revoke passports at will. Some countries, such as the former Soviet Union, further required that their citizens, and sometimes foreign travelers, obtain an exit visa to be allowed to leave the country. Currently, some countries require that foreign citizens have valid visas upon leaving the country if they needed one to enter. For example, a person who overstayed a visa in Czech Republic may need to obtain an exit visa. In Russia, the inconvenience goes even further as the legislation there does not formally recognize residency permits as valid visas; thus, foreign citizens lawfully residing in Russia need to obtain "exit-entry" visas in order to do a trip abroad. This, in particular, affects foreign students, whose original entry visas expire by the time they return home. Citizens of the People's Republic of China who are residents of the mainland are required to apply for exit and entry endorsements in order to enter the Special Administrative Regions of Hong Kong and Macau (and SAR residents require a Home Return Permit to visit the mainland). Since 2016, residents of the Xinjiang Uyghur Autonomous Region have been required to deposit their passports with the police. Each trip abroad