H-2B temporary skilled and unskilled workers: Depends on the labor certification and the proposed period of employment, plus a period of up to ten days before the validity period of the H-2B petition begins and ten days after it ends. Initial maximum of 12 months, with extensions of up to a year possible, limited by an overall maximum of three years.

Some countries apply the principle of reciprocity in their visa policy. A country's visa policy is called 'reciprocal' if it imposes visa requirement against citizens of all the countries that impose visa requirements against its own citizens. The opposite is rarely true: a country rarely lifts visa requirements against citizens of all the countries that also lift visa requirements against its own citizens, unless a prior bilateral agreement has been made.
With some countries, the validity of a visa is not the same as the authorised period of stay. The visa validity then indicates the time period when entry is permitted into the country. For example, if a visa has been issued to begin on January 1 and to expire on March 30, and the typical authorised period of stay in a country is 90 days, then the 90-day authorised stay starts on the day the passenger enters the country (entrance has to be between 1 January and 30 March). Thus, the latest day the traveller could conceivably stay in the issuing country is 1 July (if the traveller entered on 30 March). This interpretation of visas is common in the Americas.
^ "China to Start Fingerprinting Foreign Visitors". Air Canada. 31 January 2019. Retrieved 7 July 2018. Effective April 27, 2018, border control authorities at all of China’s ports of entry, including its airports, will start collecting the fingerprints of all foreign visitors aged between 14 and 70. Diplomatic passport holders and beneficiaries of reciprocal agreements are exempted..
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A visa allows a foreign citizen to travel to a U.S. port-of-entry (generally an airport) and request permission to enter the United States. A visa does not guarantee entry into the United States. The Department of Homeland Security (DHS), U.S. Customs and Border Protection (CBP) officials at the port-of-entry have authority to permit or deny admission to the United States. If you are allowed to enter the United States, the CBP official will provide an admission stamp or a paper Form I-94, Arrival/Departure Record. Learn more about admissions and entry requirements, restrictions about bringing food, agricultural products, and other restricted/prohibited goods, and more by reviewing the CBP website.
Marriage visa, granted for a limited period before intended marriage or civil partnership based on a proven relationship with a citizen of the destination country. For example, a German woman wishing to marry an American man would obtain a Fiancée Visa (also known as a K-1 visa) to allow her to enter the United States. A K1 Fiancée Visa is valid for four months from the date of its approval.[12]
So, for example, someone who arrives in the U.S. with a fiancé visa (K-1) and applies for a work permit will receive one that lasts only until the 90-day termination of that person’s K-1 visa. Although it might sound like this would create problems for fiancés who plan to apply for green cards after marriage and stay in the United States, it actually doesn’t. That’s because the fiance can simply apply to adjust status as soon as they’ve gotten married, and then apply for an EAD that lasts even longer, at that time.
Some countries, such as Canada and the United States, may require the visitor to include a letter of invitation with their travel visa application. A letter of invitation is a formal letter from the person you intend to visit stating that they are inviting you to visit them in that country. Invitation letters help travel authorities vet potential visitors by making sure that a temporary visit is indeed the true nature of the visa request. Check with your intended destination’s government website for details on what needs to be included in a such a letter.

The visa or the completed Visa Waiver Program document permits non-immigrant travelers to enter upon U.S. soil only to the extent that they can present themselves to a determining Department of Homeland Security official. The official and his work station are euphemistically referred to as "the gate." The document that demonstrates permission to enter the country has been granted is universally called an I-94 -- the proper term is Arrival/Departure Record -- and it is issued by the determining officer at the gate. In simple terms, the visa allows the traveler to knock on the door; the I-94 card is proof the Department of Homeland Security has allowed them to come in. The I-94 determines the time limit of the stay, and the restrictions imposed upon the visitor while in the U.S. Visas do not allow any stay of any length within the U.S.
Azerbaijan also strictly bans any visit by foreign citizens to the separatist region of Nagorno-Karabakh[138] (the de facto independent Republic of Artsakh), its surrounding territories, and the Azerbaijani exclaves of Karki, Yuxarı Əskipara, Barxudarlı, and Sofulu which are de jure part of Azerbaijan but under the control of Armenia, without the prior consent of the government of Azerbaijan. Foreign citizens who enter these territories will be permanently banned from entering the Republic of Azerbaijan[139] and will be included in their "list of personae non gratae".[140] As of 21 February 2019, the list mentioned 803 people.
On what basis would she be able to adjust status once in the U.S.? Of course she shouldn't do this. If immigration officials even *think she entered the U.S. on a tourist visa with the intention of immigrating, she'll never adjust status. It's never advisable to enter the U.S. on a tourist visa with the intent of immigrating. http://www.dixonimmigration.com/index.php?pid=2
In the absence of specific bilateral agreements, countries requiring passports to be valid for at least 6 more months on arrival include Afghanistan, Algeria, Anguilla, Bahrain,[119] Bhutan, Botswana, British Virgin Islands, Brunei, Cambodia, Cameroon, Cape Verde, Cayman Islands, Central African Republic, Chad, Comoros, Costa Rica, Côte d'Ivoire, Curaçao, Ecuador, Egypt, El Salvador, Equatorial Guinea, Fiji, Gabon, Guinea Bissau, Guyana, Haiti, Indonesia, Iran, Iraq, Israel,[120] Jordan, Kenya, Kiribati, Kuwait, Laos, Madagascar, Malaysia, Maldives, Marshall Islands, Mongolia, Myanmar, Namibia, Nepal, Nicaragua, Nigeria, Oman, Palau, Papua New Guinea, Philippines, Qatar, Rwanda, Samoa, Saudi Arabia, Singapore, Solomon Islands, Somalia, Sri Lanka, Sudan, Suriname, Tanzania, Thailand, Timor-Leste, Tokelau, Tonga, Tuvalu, Uganda, United Arab Emirates, Vanuatu, Venezuela, and Vietnam.[121]

The United States is an open society. Unlike many other countries, the United States does not impose internal controls on most visitors, such as registration with local authorities. Our immigration law requires consular officers to view every visa applicant as an intending immigrant until the applicant proves otherwise. In order to enjoy the privilege of unencumbered travel in the United States, you have a responsibility to prove you are going to return abroad before a visitor or student visa is issued.