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In most cases, and for the sake of simplicity, the common name of a country (and not the ISO one) should be used in addresses on envelopes. In most cases, the common name is better recognized or simply easier to fit on the envelope than the official country name.
– Venezuela (the common name) and not Bolivarian Republic of Venezuela (the ISO name).
– Bolivia (the common name) and not Plurinational State of Bolivia (the ISO name).
No, there is no rule governing which postcodes are numeric and which are alphanumeric. The decision is purely conventional and up to the country concerned.
The following list contains information about postcode type per country. If a country has a single postcode, that postcode has been entered as a postcode type. There are some countries that have a numeric postcode but with a special character (or whitespace or dash "–"), such as Brazil or Czech Rep. those countries have been classified as Numeric (*).
The issue of two-character ISO codes preceding postcodes can be quite confusing. First of all, prefixing a postcode with a two-character ISO code is not – and has never been – an inter-national standard. It is up to the administration of the country concerned to decide whether two-character ISO codes should appear in the addresses or not.
Some countries dislike this practice because it disturbs their sorting routines. However, countries from some regions (e.g. Scandinavia, Benelux, Baltic countries) still use them, which is where the issue of ISO codes in addresses becomes complex. The use of ISO codes is not discouraged by the UPU either. Here is a table indicating which countries use the ISO prefix.