绿卡有签证功能吗？
美国永久居民（1），也叫绿卡持有人，没有签证功能 美国的非移民签证（2），包括F1，J1，H.M等等，这些持有人的身份是临时入境者，他们来到美国后，必须遵守入境时美国政府给予他们的入境限制条件，比如不能工作，或者在美国居留的时间不能超过规定的期限等 有些“签证”持有人的身份也是临时入境者，不过这些“签证”持有人来美后并没有受到任何入境限制（3）——只要不违反入境的实质性条件，他们在美可自由居住、就业、学习等等。这些人持有的不是“签证”，而叫“护照”。他们拥有这种可以在美国合法滞留的权利的原因并不是因为政府授予了这种权利给他们，而是因为美国政府承认他们国籍国政府赋予他们的在美自由活动权。
1) 关于这个身份的具体定义可以参见USCIS 的解释： https：//www.uscis.gov/greencard/permanent-resident-green-card 根据这个网站的介绍，一个“绿卡”持有人被称为一个合法的永久居民(lawful permanent resident， LPR)，他除了不能投票以外，在法律上有和公民一样的权力义务。如果他被政府以某种方式剥夺了这个身份或权利（例如被驱逐出境），他就变成了非法移民(alien in unlawful status)， 而不再是合法永久居民。
2) Nonimmigrant means any person not born or naturalized in the United States who is allowed to enter the United States for a specific purpose which is temporary (the duration of stay being specified by law)， and who has not been accorded a foreign nationality status under the laws of the United States [8 USC § 1101]. The classification may also include， but does not necessarily depend on whether or not the alien has been accorded non immigration status pursuant to section 101(a)(15) of the Immigration and Nationality Act， i.e.， as an immigrant having a foreign country of nationality or last legal permanent residence; as a nonimmigrant under sections 101(a)(26)， 101(a)(16)， 101(a)(20)， and other subparagraphs thereof；as a stateless refugee under section 207 of the Act， §§ 1324B and D of title IV of the Illegal Immigration Reform and Immigrant Responsibility Act of 1996， Pub.L. No.104—208， §§ 308(e)(2)(I)， 303(c); and as an asylum applicant admitted according to section 208 of the Act， Pub.L. No.104–208， §§ 308(A)， (C)， and amended by sections 506(a) and 509， Anti-Terrorism and Effective Death Penalty Act of 1996. For this definition， see US Citizenship and Immigration Services Glossary at http：//www.uscis.gov/glossary #：p=non_immigrant “Nonimmigrant” 的字面意思是“不属于居民的”，因此中文里常翻译为“非移民”。
3) A "visa" is issued by a government to allow a foreign citizen to travel to that nation's territory， subject to conditions set out therein. In some countries it can also be used as proof of identity or citizenship when required by local authorities. It grants the holder certain rights related to transit through their territory： usually entry into the country itself