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Here you have the global parameters that are used by transformations.

Names that are stored in ID3V2.3.0 are supposed to be either 8bit Latin-1 or 16bit Unicode UTF16 (ID3V2.4.0 has 2 more formats, of which only one is currently supported by MP3 Diags.) However, some tools use the 8bit option with other locales on frames that are declared Latin-1, when they should be using UTF16. Although those tools display the names correctly, others don't, especially if the file is moved to another computer. The reverse might happen as well: correct files using Latin-1 might appear to have incorrect frames, because they are interpreted according to some other locale. There is a transformation that does conversions from 8bit to UTF16, and it is here that you specify the locale to be used in the process.

The locale setting is also used by another transformation, which copies song information from ID3V1 tags to ID3V2.

By not allowing more than 1 image per file you make sure that all the players see the same image. OTOH, if you have files containing more than 1 image, you'll probably want to keep them all, in which case the "Keep a single image" option should be unchecked.

The case transformations are not perfect, and there are no plans to improve them much. The biggest issue is probably not being aware of most proper names.