A visa generally gives non-citizens clearance to enter a country and to remain there within specified constraints, such as a time frame for entry, a limit on the time spent in the country, and a prohibition against employment. Some countries do not require a visa in some situations, such as a result of reciprocal treaty arrangements. The possession of a visa is not in itself a guarantee of entry into the country that issued it, and a visa can be revoked at any time. A visa application in advance of arrival gives the country a chance to consider the applicant's circumstance, such as financial security, reason for applying, and details of previous visits to the country. A visitor may also be required to undergo and pass security and/or health checks upon arrival at the border.