Question: ISO 3166-1 alpha-2 is a widely used country code system, along with what three-letter system?

Context articles: ISO 3166-2:SJ
ISO 3166-1 alpha-2
ISO 3166-2:RU
ISO 3166-1 alpha-3
.bq
.cw
UN M.49
ISO 3166-2:TW
Svalbard and Jan Mayen
.bl

ISO 3166-2:SJ is the entry for Svalbard and Jan Mayen in ISO 3166-2, part of the ISO 3166 standard published by the International Organization for Standardization (ISO).
 The standard defines codes for names of principal subdivisions of all countries coded in ISO 3166-1.
 Svalbard and Jan Mayen does not exist as an administrative region, but rather consists of two separate parts of Norway under separate jurisdictions—Svalbard and Jan Mayen.
 Further subdivision for Svalbard and Jan Mayen occurs under Norway's entry, , namely NO-21 for Svalbard and NO-22 for Jan Mayen.
 There are currently no ISO 3166-2 codes for Svalbard and Jan Mayen.
ISO 3166-1 alpha-2 codes are two-letter country codes defined in ISO 3166-1, part of the ISO 3166 standard published by the International Organization for Standardization (ISO), to represent countries, dependent territories, and special areas of geographical interest.
 They are the most widely used of the country codes published by ISO (the others being alpha-3 and numeric), and are used most prominently for the Internet's country code top-level domains (with a few exceptions).
 They were first included as part of the ISO 3166 standard in its first edition in 1974.
ISO 3166-2:RU is the entry for Russia in ISO 3166-2, part of the ISO 3166 standard published by the International Organization for Standardization (ISO), which defines codes for some of the names of the principal subdivisions (e.g., provinces or states) of all countries coded in ISO 3166-1.
 ISO 3166-2:RU contains no codes for the Russian-administered Republic of Crimea and Federal City of Sevastopol, which are internationally recognized as part of Ukraine and do have codes in .
ISO 3166-1 alpha-3 codes are three-letter country codes defined in ISO 3166-1, part of the ISO 3166 standard published by the International Organization for Standardization (ISO), to represent countries, dependent territories, and special areas of geographical interest.
 They allow a better visual association between the codes and the country names than the two-letter alpha-2 codes (the third set of codes is numeric and hence offers no visual association).
 They were first included as part of the ISO 3166 standard in its first edition in 1974.
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bq is designated—but not in use—as the Internet country code top-level domain (ccTLD) for "Bonaire, Sint Eustatius and Saba" (the Caribbean Netherlands) following the assignment on December 15, 2010, by the ISO 3166 Maintenance Agency of BQ as the ISO 3166-1 alpha-2 to the area.
 This decision followed the dissolution of the Netherlands Antilles and new status of the Caribbean Netherlands as public bodies of the Netherlands on October 10, 2010.
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cw is the Internet country code top-level domain (ccTLD) for Curaçao.
 It was created following the decision on December 15, 2010 by the ISO 3166 Maintenance Agency to allocate CW as the ISO 3166-1 alpha-2 code for Curaçao.
 This decision followed Curaçao's new status as an autonomous country within the Kingdom of the Netherlands on October 10, 2010.
 The University of Curaçao, which already was the sponsor for .
an was designated as the sponsoring organization.
 Registration of .
cw domains is available from 1 February 2012.
UN M.49 is a standard for area codes used by the United Nations for statistical purposes, developed and maintained by the United Nations Statistics Division.
 Each area code is a 3-digit number which can refer to a wide variety of geographical, political, or economic regions, like a continent, a country, or a specific group of developed or developing countries.
 Codes assigned in the system generally do not change when the country or area's name changes (unlike ISO 3166-1 alpha-2 or ISO 3166-1 alpha-3), but instead change when the territorial extent of the country or area changes significantly, although there have been exceptions to this rule.
ISO 3166-2:TW is the entry for Taiwan in ISO 3166-2, part of the ISO 3166 standard published by the International Organization for Standardization (ISO), which defines codes for the names of the principal subdivisions (e.g., provinces or states) of all countries coded in ISO 3166-1.
 Due to the political status of Taiwan in the United Nations, the ISO 3166-1 full name is Taiwan, Province of China.
Svalbard and Jan Mayen (Norwegian: "Svalbard og Jan Mayen" , ISO 3166-1 alpha-2: SJ, ISO 3166-1 alpha-3: SJM, ISO 3166-1 numeric: 744) is a statistical designation defined by ISO 3166-1 of two parts of Norway under separate jurisdictions—Svalbard and Jan Mayen.
 While the two are combined for the purposes of the International Organization for Standardization (ISO) category, they are not administratively related.
 This has further resulted in the country code top-level domain .
sj being issued for Svalbard and Jan Mayen, and .
 The United Nations Statistics Division also uses this code, but has named it Svalbard and Jan Mayen Islands.
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bl is an assigned Internet country code top-level domain (ccTLD) that was to be created for Saint Barthélemy, following the decision on 21 September 2007 by the ISO 3166 Maintenance Agency to allocate BL as the ISO 3166-1 alpha-2 code for Saint Barthélemy.
 This decision followed Saint Barthélemy's new status as an Overseas collectivity of France which took effect on 15 July 2007.
 Currently Saint Barthélemy uses Guadeloupe's ccTLD, .
gp and France's ccTLD, .
fr.