Many countries issue inflation stamps that have a letter or number indicating a tariff. Spain uses tariffs A (National), B (Europe) and C (World) for post cards and letters up to 20 grams. The Netherlands has stamps with either a 1 or 2. These are the weight steps. If a priority label is attached, it is for international postage, otherwise, it is for inland mail.
The UK has tariffs 1 and 2 for first or second class mail. These exist as normal or large; the latter for larger than normal items. They also had an E-tariff for letters to Europe. But xenophobia got the better of the Brits. Many thought the letter stood for "European Union." So, it was abolished and these stamps now have a value indicator again. At one time Great Britain also issued "Worldwide" stamps.
France also has a range of nvi (no value indicated) stamps. - As I wrote this, vayolene posted some of the French stamps.

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In the case of these Transnistrian stamps, the issue is that the stamps are not recognised internationally. On top of that, there are also many bogus issues for that territory.